Require colors for logging errors
Send debug messages to stderr

Send debug messages to stderr to allow both errors and debug noise to
be filtered out (e.g. redirecting stderr).
Style fix for consitency
sniff/loadUrl: Change argument signature

We'll now pass an options object into loadUrl rather than manually
specifying every possible configuration value.
sniff/loadUrl: Add support for setting a custom viewport size

Closes #50.
Merge branch 'add/viewport-option' of https://github.com/stephenmathieson/pa11y into develop
Version 1.6.0
Document the viewport option
Expose the viewport option to the command line