Add minzoom override

[GretaCB]

In the case of a happytiff, no need to include semi-empty tiles for zoom levels below the happytiff's preferred zoom level (aka "happy zoom"). Add a minzoom override option to specify happyzoom.

mapbox-tile-copy will set the final minzoom to the value passed in via the options object and ignore the minzoom passed in via mapnik-omnivore

• Add a --minzoom option within bin/mapbox-tile-copy cli
• Add some validation of the minzoom value (numeric)
• Add tests

cc @camillacaros @rclark

[GretaCB]

Merged #37. Closing