Feature: VimwikiVar to list, get and set variables

[tinmarino]

This PR comes with a big code refaction in vars.vim. Especially for syntaxlocal variables.

VimwikiVar outputs

VimwikiVar Completion

without arglead, with arglead and for the second argument, the value of the variable

In a few commits, more refaction, features and syntaxlocal variables names, tests => a permanent solution for #892 will be provided and #894 may be closed.

[tinmarino]

It should be good for a first shot. Many exported but non documented variables such as these are not ready to use:

        \ 'bold_match': {'type': type(''), 'default': '\%(^\|\s\|[[:punct:]]\)\@<=\*__Text__\*\%([[:punct:]]\|\s\|$\)\@='},
        \ 'bold_search': {'type': type(''), 'default': '\%(^\|\s\|[[:punct:]]\)\@<=\*\zs\%([^*`[:space:]][^*`]*[^*`[:space:]]\|[^*`[:space:]]\)\ze\*\%([[:punct:]]\|\s\|$\)\@='},
        \ 'bullet_types': {'type': type([]), 'default': ['-', '*', '#']},
        \ 'header_match': {'type': type(''), 'default': '^\s*\(=\{1,6}\)=\@!\s*__Header__\s*\1=\@!\s*$'},
        \ 'header_search': {'type': type(''), 'default': '^\s*\(=\{1,6}\)\([^=].*[^=]\)\1\s*$'},

Note that the exported format will often look like that: it permits to group pre and post markers.
Alos, a solution to group search and match must be designed. And this kind of format can be extended to a list if many format types are allowed.

        \ 'tag_format': {'type': type({}), 'default': {
        \   'pre': '^\|\s',
        \   'pre_mark': ':',
        \   'in': '[^:''[:space:]]\+',
        \   'sep': ':',
        \   'post_mark': ':',
        \   'post': '\s\|$',
        \   'conceal': 0, 'cchar':''}},

And after all that, maybe we can let user choose between different link formats as #892