Add a test framework based on vader.vim

[l1laxS]

This pull-request adds a test framework based on vader.vim.

I've setup a dedicated minimal .vimrc to isolate test runs from the wikis specified in the user g:vimwiki_list. A detailed explanation of how to run the tests can be found in tests/README.md.

I've set up only a few tests for:

• zettel creation
• replacement of highlighted text with link (markdown)
• zettel creation with/without user defined template and frontmatter

I hope this can be a first step in helping others adding new functionalities, without breaking anything.

[michal-h21]

Wow, thanks! I didn't know about vader.vim, but it looks interesting.