Adding support for an ignore file.

I love foodcritic. It's an awesome linter and very useful to maintain a quality standard in a chef repo shared among many people.

Very rarely, we get a few foodcritic warnings that we wish to ignore... be it from issues we do not wish to fix (for whatever reason) or occasional false positives. 

Since we use foodcritic as part of our continuous delivery pipeline, we don't want our build to fail for these warnings, but we don't want to ignore an entire class of rules. It was with this in mind that I added the ability to pass an "ignore file" via the command line. An example file looks like this:

```
## foodcritic ignore file

# omitted because refactoring this makes it LESS elegant
FC004: Use a service resource to start and stop services: cookbooks/runit/recipes/default.rb:22
# omitted because it's too short and specific to make a library
FC014: Consider extracting long ruby_block to library: cookbooks/mongodb/recipes/mms-agent.rb:37
# omitted because contains false positives
FC017: LWRP does not notify when updated: cookbooks/python/providers/pip.rb
```

It's basically the output of a foodcritic run. You can choose to ignore a warning on a specific line, or omit the line number to ignore the rule across the entire file (c.p. the last line in the above example). Comments and newlines are ignored as expected.

I believe that this is a practical change, simply because foodcritic is often used in build pipelines and it's suboptimal to have to make changes to "trick" foodcritic into ignoring false positives.

Also, I know it's not perfect, but I figured I'd share it with you guys to collect feedback and to see what it would take to get it into master.

Thanks!
-Dana