Support passing arbitrary arguments to shellcheck

[ian-h-chamberlain]

Semi-related to #14

This set of rules looks really promising! I was able to get it up and running easily, but I noticed there doesn't seem to be any way to pass arguments to shellcheck, which can be useful for a variety of reasons:

• --color= argument
• --include / --exclude to control which lints are run
• --format= for alternative output formats

Would you consider exposing an args=[] parameter on the main rule for these purposes? I can probably work around it by defining my own rule in the meantime, but this seems like something that others would probably find useful as well.