[feat] Add cache purge support to the flarectl CLI.

[elithrar]

• The upcoming Purge by Host functionality has been added to the library
• flarectl now has a purge sub-command for purging from cache.

e.g

# Purge by host (upcoming)
flarectl zone purge --zone=example.com --hosts="repeater.example.com"

# Purge everything 
flarectl zone purge --zone=example.com --everything

# Purge by filename 
flarectl zone purge --zone=example.com \
  --files="http://repeater.example.com/styles.css,http://repeater.example.com/javascript.js"

[jamesog]

LGTM. We really need to look at switching out the CLI library for something a little nicer.

[elithrar]

@jamesog That's my 🎄🎄🎄 Holiday Project ® 🎄🎄🎄