Add support for 'con requests start'

[zanerock]

con requests start <item ID> [branch-label=<branch label>]

'start' is a CLI verb. Requires an item ID. Implies:

con requests PATCH <item ID> [branch-name=<item ID>[-<branch label>] assignee=<users ID>

where 'users ID' is determined by the CLI environment settings.

Implements: DogFoodSoftware/conveyor-core#5