v2.0.0: New major version of aspeak

v2.0.0 finally arrived.

Changes:

• Remove old Synthesizer API
• Implement new functional API
• Migrate the CLI to new API
• Better CLI help message
• More value formats for argument pitch and rate
• Argument checks for pitch/rate/format
• Many internal refactors
• Configure pylint
• Examples for the new API
• Documentation for new API
• Better documentation
• Export error types to top level module