Allow environment variable filtering

[jkeyes]

This can prevent some variables being sent to Exceptional. This feature will be useful to people who keep their configuration in environment variables (e.g. Heroku). Devs can decide to prevent api keys, and the like from being displayed in Exceptional.

For example, the following code would prevent the AWS_SECRET and AWS_ACCESS variables from being sent in the exception environment to Exceptional:

EXCEPTIONAL_ENV_EXCLUDE_VARS = ('AWS_SECRET', 'AWS_ACCESS')

I haven't included any tests yet, but the change itself is straight-forward.