Allow uppercase_first_letter for underscore function

[abbbi]

underscore function will always alter variable assigments True/False, like so:

>>> inflection.underscore("snakeCase = True")
'snake_case = true'

i think it would make sense to have something like uppercase_first_letter here too?

[luckydonald]

Same with their examples like

camelize('active_model/errors') 
   =>    "ActiveModel::Errors"

>>> camelize('active_model/errors')
'ActiveModel/error'

I however would expect
'ActiveModel/Error' with an capital E here.