Implement enough functionality to support vim-gutentags

[jberryman]

I would like to use vim-gutentags to manage tag creation, but hasktags doesn't quite support it. See the requirements section:

https://github.com/ludovicchabant/vim-gutentags/blob/master/doc/gutentags.txt#L66

This means:

• implementing --exclude in a ctags-compatible way
• possibly fixing --append (see What does the warning on --append about "not sorted" mean? Does append actually work? #67 ... I don't understand the situation there)

There is a shim project that I haven't yet looked closely at here: https://github.com/rob-b/gutenhasktags (it may not even work). Obviously it would be nice if that wasn't necessary.

I'd like to help if that's of interest.

[jhenahan]

For sure! I'm actually on vacation next week, so I can definitely set aside some time to look into it. Happy to help with PRs, too, if you want to work on one.