Add development build channel option

[meloniq]

Option that would allow the plugin to offer development builds updates instead of only tagged releases.

Potential solution:

• Updater plugin have a new option to choose channel, and passes additional arg to AppThemes Updates API eg. 'channel' => 'dev'
• Jenkins after generating development build, updates also product_dev_version meta on .com site (on .com use WP Ajax and validate request with secret key?!)
• AppThemes Updates API checks 'channel' param, and when 'dev' uses product_dev_version meta value to compare versions, and returns download url with the '/dev' suffix (eg. https://api.appthemes.com/themes/download/classipress/dev)