hglib: handle cases when user didn't provide any input

spec: test runcommand_co()

docs: missing ticks

hglib: make inner loop in runcommand() use a coroutine Then it's possible to do something nontrivial using the more low-level client.runcommand_co().

docs: add a readme

rockspecs: more detailed description

hglib: commands now can receive input

spec: exit codes are not very useful, check them last Error and output channels usually have readable messages explaining what went wrong, they need to be checked first.

spec: assert(io.open()) to fail early

hglib: have an error message for unexpected data on required channel