refactor: fixed gulp-util deprecations

[cebreus]

• Replaced gulp-util.log with fancy-log
• Replaced gulp-util.colors with ansi-colors
• Replaced underscore with flat-options
• Added check if options.to is a single email or array of emails
• Removed _subjectFromFilename; use basename if subject not provided

Picked from 158c48fc75d0f86e643095320ae7b899485b54001 by justin713

[cebreus]

Hi @fritx, please consider merging.