cmd step: support ~ & env

[yaythomas]

pypyr.steps.cmd doesn't support ~ expansion or $ENV vars in its cmd input.

Easy enough to add. Then operators won't have to go and invoke the more inefficient pypyr.steps.shell just for doing something like:

- name: pypyr.steps.cmd
   in:
     cmd: cp ~/mypath ~/myoutpath

Note that $ENV expansion supports formats $ENV and ${ENV}. It leaves unrecognised tokens alone, meaning pypyr can do a {substitution} format afterwards without breaking anything.

[yaythomas]

The Python ~ expandUser func is pretty inefficient, and it'd have to run for each arg in cmd inputs args. . .

Might be better to create a step to write {HOME} into context and use that as a standard formatting expression.