Add support for custom messages

[astorije]

First of, is this a desired feature?

@ulrikhindoe started working on this at #85 but it never got in simply because I wasn't responsive enough.
If anyone wants to help with this, I'll be more careful :)

[aburtsev]

Hey @astorije,
Is your request for help relevant?
Thanks for your work. I am using your library and want to fix this inconsistency with the original Chai API.
I ask because I want to make sure you are open to review of PR