Another big refactor, I'm sorry :( this should be the last one for now..
If needed I can split this up, but all changes depend on each other anyway.
removes global flag values in cmd/flags (closes#158)
introduces TeaContext, a wrapper around cli.Context
simplifies InitCommand() call signature to return TeaContext
checks command requirements on TeaContext via ctx.Ensure()
Since #285 all commands can be run outside of a git repo.
For some commands this was desired, for others requirements are too loose now.
This readds requirement checks in a more flexible way.
Could be considered a bugfix and should go into v0.6.0
added this to the v0.6.0 milestone 2 years ago