Add help messages similar to virtualenvwrapper

[Sekuraz]

mkvirtualenv --help should print a help message and not create a virtualenv named --help. The same applies to almost all commands in compat_aliases.fish

[daisylb]

This is a good idea. If anyone feels like contributing, I'd accept a pull request that implements this so long as it's implemented in the actual __vf functions in core, and not just in compat_aliases. If not, I'll probably add it myself when I have spare time.

[justinmayer]

I implemented vf new --help, and thus by extension mkvirtualenv --help, via #159. If someone would like to refer to how I implemented it and do the same for other functions, I would be happy to review and merge said contributions.