Mercurial support

[ghost]

the oh my fish agnoster theme used to have a seperate version with mercurial support https://github.com/oh-my-fish/theme-agnoster-mercurial which then got moved into the default one eventually https://github.com/oh-my-fish/theme-agnoster

it would be great if this one could have mercurial support aswell.

[hauleth]

I am not a Mercurial user and I know only a little of it, so I would welcome PR as personally I lack knowledge.

[ghost]

I've never written anything for fish. If someone feels like doing it, heres a link to the file that (i think) handled the mercurial support in the agnoster theme for oh-my-fish version that might be helpful.

https://github.com/oh-my-fish/theme-agnoster/blob/master/fish_prompt.fish