Discourage `f` or `x` prefix on specs

[SimenB]

Instead of doing fit, xit etc, one should do it.only and it.skip. Same for describe.

Should also be fixable

[ranyitz]

I can also take this issue as it closely related to #12

name suggestions:

1. prefer-only-skip
2. no-fx-prefix

[SimenB]

I like the first one :) @thymikee?

[thymikee]

@SimenB yup, I'm on your side on this :)