Add lyrics export from Artist class to .txt or .json formats

[johnwmillr]

This PR closes #13 and #11. The Artist class now has a save_lyrics() method that gives the user the option to export all lyrics in an artist object into either .json or .txt format. The PR also includes unit tests for both export options. I guess to fully address #11 I should add a save_lyrics method to the Song class, but I'm not going to do that now. I will do that later.

Usage:

artist = api.search_artist("Andy Shauf", max_songs=3)

# Method includes optional params: format, filename, overwrite, skip_duplicates
# Default format is .json
artist.save_lyrics()