Adding support for role-based invokes

[Virviil]

All Bootleg mix tasks supports environment name as first parameter:

mix bootleg.ping db

except bootleg.invoke

This PR adds support for this feature:

mix bootleg.invoke my_custom_task build

[rjanja]

Hi @Virviil thank you for opening this PR. I think it's close, but needs some changes:

• Wording: environment and roles are very separate things. To be clear this adds support for environment-specific mix task invocations.
• Order of parameters: in other mix bootleg tasks, one can specify the environment as the first argument e.g. mix bootleg.ping qa2. As implemented here it would be mix bootleg.invoke build qa2 which may lead to confusion.
• We can't assume the head of the tail of the list is going to be an environment

[rjanja]

Closing and tracking this via #221