Add an option for a banner

A banner is a string prepended to the specified output file.

Closes #45.
Closes #62.
Merge pull request #62 from drublic/banner

PR: Add an option for a banner (#45)
0.5.0
Update changelog, readme, etc
Mention that `banner` isn't compatible with `sourcemap`
Log successful file creation.
Merge pull request #64 from MayhemYDG/patch-1

Log successful file creation.