cmd.go: let env var CRANE_FILE specify the manifest file to use

file is chosen as
1.  commandline option, if specified
2.  CRANE_FILE environment var, if exists
3.  default list
Add targets flag to limit the containers the cmd should apply to

Sometimes you want to operate on only some (perhaps just 1) of the
containers specified in your config.  This feature allows that.

One example use-case will be to run a shell in the same environment you'd
normally run a server (eg to do some manage.py commands for a django app).
this use-case will be better with another feature i'm about to write
that will allow you to specify some containers in the manifest that
won't be automatically launched by start/run.
Update help and README.md to describe priority for config
print long help for `crane help`

the cobra package only prints usage info when you invoke just
`crane help`.  I think that cobra ought to be patched to print
both the help and the usage for the root cmd in this case.  but, easy
to override by providing our own help command, whose implementation
is the cobra HelpFunc() modified to also print the Help not just Usage
Add manual start containers

these only start if specified using the --target flag
Merge branch 'exec_for_some_containers'

* exec_for_some_containers:
  Add manual start containers
  Add targets flag to limit the containers the cmd should apply to