Custom UUID char

[shfattig]

Allowing the user to modify the # prepended to the {uuid} at the end of todo items, therefore providing the potential to deconflict with other tools which may reserve the # symbol for something such as tags. All the user has to do is set g:taskwiki_uuid_char in their configs

mapped to issue #436

[shfattig]

I updated the tests similarly to the rest of the code but haven't actually run them (I know, I know - but it's *working*!)
However, I am thinking that it's likely the tests should run without vim in the loop, in which case reworking the test updates is likely needed, and potentially some new cases. I'll let @maintainers look over my initial implementation before jumping into that though.

[shfattig]

I "tested" many different symbols and characters, and the only one I found that had issues was \ (understandably) -- is the best way to handle this just by preventing this character from being passed through at all?