[refactor] This is not a filetype plugin - remove syntax highlighting

[lervag]

As already discussed before in #8, #9, and #12, it makes a lot of sense to decouple the wiki part of this plugin from the syntax highlighting. Syntax highlighting is already provided by other plugins, such as:

• https://github.com/plasticboy/vim-markdown
• https://github.com/gabrielelana/vim-markdown
• https://github.com/vim-pandoc/vim-pandoc-syntax

We should therefore remove this from wiki.vim and focus on link actions and similar. There is a lot of obvious potential, e.g. to support different styles of links in a natural and dynamic manner, but the first step is to make clear what the plugin should provide.

In this regard, here is a short list of actions:

• Remove syntax highlighting from wiki.vim
• Allow to load wiki.vim for other filetypes, e.g. markdown files (perhaps with g:wiki_extensions variable)
• Add section on various Markdown syntax plugins in wiki.vim docs, preferably with a recommendation on the most mature syntax highlighting plugin

[lervag]

This issue was solved in bc6f0b4 (and some later commits).