Feature: Optionally disable todo propagation to parents/children (Issue #954)

taskwiki integrates vimwiki with taskwarrior, and in doing so changes
the semantics of checkboxes a bit:

    * [ ] Install Taskwiki   |   pending task
    * [X] Install Taskwiki   |   completed task
    * [D] Install Taskwiki   |   deleted task
    * [S] Install Taskwiki   |   started task
    * [W] Install Taskwiki   |   waiting task

It's still desirable for vimwiki to automatically insert `* [ ]` on
`i_<CR>`, `o` and `O` and to syntax highlight all these five as
checkboxes, so I have this in my .vimrc:

    let g:vimwiki_listsym_rejected = 'D'
    let g:vimwiki_listsyms = ' WSX'

but it results in undesirable behaviour with task hierarchies: when I
add a new subtask (using `i_<CR>`, `o` or `O`)
or mark a subtask done, the parent's checkbox is updated to reflect its
overall completion, to one of ` `, `W`, `S` or `X`, depending on
subtasks completion. This makes little sense in taskwiki. One usually
doesn't want to touch the "parent" task in taskwarrior until the
"subtasks" are done. Setting

    let g:vimwiki_listsym_rejected = 'W'
    let g:vimwiki_listsyms = ' SX'

results in slightly less illogical behaviour, but it still assumes that
(1) all subtasks are visible (not necessarily true in taskwiki) and
(2) that it's a parent/subtask relationship, not a dependency
relationship (not true in taskwarrior, questionable in taskwiki).

This commit adds an option to disable this behaviour.

doc/vimwiki.txt

@@ -1880,9 +1880,10 @@ parent items: >
    * [ ] Add highlighting to list item boxes.
    * [ ] Add [ ] to the next list item created using o, O or <CR>.
 
-Parent items should change when their child items change. If not, use
-|vimwiki_glr|. The symbol between [ ] depends on the percentage of toggled
-child items (see also |vimwiki-listsyms|): >
+Parent items should change when their child items change unless disabled via
+|vimwiki-option-listsyms_propagate|. If not, use |vimwiki_glr|. The symbol
+between [ ] depends on the percentage of toggled child items (see also
+|vimwiki-listsyms|): >
     [ ] -- 0%
     [.] -- 1-33%
     [o] -- 34-66%
@@ -2674,6 +2675,16 @@ You can set it to a more fancy symbol like this:
    let g:vimwiki_list = [{'path': '~/path/', 'listsym_rejected' = '✗'}]
 
 
+*vimwiki-option-listsyms_propagate*
+------------------------------------------------------------------------------
+Key                 Default value     Values~
+listsyms_propagate  1                 0, 1
+
+Description~
+Set this option to 0 to disable propagation of todo list item status to
+parents and children.
+
+
 *vimwiki-option-auto_tags*
 ------------------------------------------------------------------------------
 Key             Default value     Values~
@@ -3831,6 +3842,7 @@ Contributors and their Github usernames in roughly chronological order:
     - Benney Au (@chinwobble)
     - David Sierra DiazGranados (@davidsierradz)
     - Daniel Moura (@dmouraneto)
+    - Tomáš Janoušek (@liskin)
 
 ==============================================================================
 16. Changelog                                              *vimwiki-changelog*
@@ -3842,6 +3854,8 @@ http://code.google.com/p/vimwiki/issues/list. They may be accessible from
 https://github.com/vimwiki-backup/vimwiki/issues.
 
 New:~
+    * Feature: #954: Add option |vimwiki-option-listsyms_propagate| to disable
+      todo propagation to parents/children
     * Issue #1009: |foldmethod| syntax works for markdown (|g:vimwiki_folding|)
       Also the VimwikiHeader1Folding (1..60 regions support end-of-file `/\%$`
       as end maker