Add ShellOption for easier options writing

This abstracts away option writing such that users no longer have to
understand scopt semantics. This adds a ShellOption class and a
HasShellOptions trait for something which provides one or more
ShellOptions.

This refactors the FIRRTL codebase to use this style of option
specification.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>