Allow a user to exclude certain JavaScript files

Allows a user to configure hound to ignore specified JavaScript files.
This follows the "jshintignore" convention. If a .jshintignore file is
found and the user has not overwritten the path in the hound
configuration, it will automatically be used.

Otherwise, a user can specify the path to a file which specifies which
files to exlude during linting.

Example configuration:

```yaml
java_script:
  enabled: true
  ignore_file: ".javascript_ignore"
```

.javascript_ignore
```
app/assets/javascripts/**.js
``
Access owner information from PullRequest
Custom thoughtbot configuration for rubocop
thoughtbot specific config for Coffeescript
Refactor default_configuration

Have base class handle logic behind thoughtbot default config vs hound
default config
thoughtbot specific config for JS linting
Refactor configuration loading in specs into helper
Extract DefaultConfigFile class
Fix up jshint ignore specs
Fix spelling of FILENAME constant