Implementation of usage() that respects width and indent (#65)

[egnha]

Resolves #65.

• Output is now bounded by width
• New boolean argument indent.by.FUN toggles indentation by function-name + (
• New string argument fail controls whether to generate a warning, stop, or continue silently, if the width constraint is unfulfillable (warning/stop message shows all overflowing lines)
• Unit tests for usage() cover width and indentation behavior in several thousand cases, in addition to the behavior of the new arguments
• In usage(), tidy_source() is replaced by tidy_usage()
• Compared to the previous implementation, typical runtime is halved

[yihui]

Thanks! This is so awesome!

[egnha]

Thanks, happy to contribute.