Add ability to pass options to firtool

Add an annotation, FirtoolOption, that can be used to pass a command
line option directly to firtool.  More than one FirtoolOption can be
specified.  Add a test that shows using this to enable case statement
emission through a "--lowering-option".

Fixes #12.

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