Add option g:vimwiki_commentstring to disable vimwiki commentstring

[BirgerNi]

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By default the commentstring ist set to %%%s. For markdown documents which should be exported to html via pandoc a commentstring like <!--\ %s\ --> may be more suitable. This PR provides the option g:vimwiki_commentstring to turn off vimwiki commentstring. The option ist turned on by default to mainain backwards compatibility.

[tinmarino]

Nice, I did not know this feature.

So you let g:vimwiki_commentstring = 0 and set commentstring='<!-- %s -->
You think about what I think ? Why don't you just let g:vimwiki_commentstring = '<!-- %s -->' and null string to prevent vimwiki setting the vim var and unset to keep default behavior (which I dont like by the way).
In other words, let the option give the chance for the user to add some customization.
Would you mind adding some 3 regression tests ?

All that said, thank you for the contribution.

[BirgerNi]

Actually for me commentstring is set by vim-pandoc plugin so I'm just looking for a way to turn it off :-)

Anyway, you mean something like this:

if !exists('g:vimwiki_commentstring')
    setlocal commentstring=%%%s
elseif g:vimwiki_commentstring != ""                                                                                                                       
    setlocal commentstring=g:vimwiki_commentstring
endif

Unfortunately with let g:vimwiki_commentstring='<!-- %s -->' in my .vimrc I'm stuck with this error:

E537: 'commentstring' must be empty or contain %s

Do you have an idea?

[tinmarino]

Do you have an idea?

Yes (sorry for the delay)
setlocal commentstring, like any set command, should not get a variable like g:vimwiki_commentstring but a verbatim string like <!-- %s --> (without the quotes)

So embed it in an execute command

if !exists('g:vimwiki_commentstring')
    setlocal commentstring=%%%s
elseif g:vimwiki_commentstring != ""                                                                                                                       
    execute 'setlocal commentstring=' . g:vimwiki_commentstring   " <----- Replace this line
endif

will work

PS: Also you can (recomended):

let &l:commentstring=g:vimwiki_commentstring

which set the local values without the overshot of execute

[tinmarino]

Then please:

1. Add test
2. Rebase

[tinmarino]

It's perfect !
Sorry for the travis fails: my bad: I change the test format, I'll reformat your test.

[tinmarino]

I changed a little bit the code: Commit: 5651e74

1. We have a standardized way to declare variables (so they are all in the same place): have a look at vars.vim; 'commentstring': {'type': type(''), 'default': '%%%s'},
2. We do not use Include: vader_includes/vader_setup.vader

Your feature is added in the dev branch, good job, see you in an other PR