Add a pushd context

[ppannuto]

I find that I write this little boilerplate all the time and I think
other sh users would find it useful as well. This simply creates a
context to run commands in another directory and ensures that the
original working directory will be returned to.

Usage:

from sh import pushd, pwd

pwd()
with pushd('/tmp'):
    pwd()
pwd()

[amoffat]

thanks Pat. merged into release-1.10. i generalized it a bit more so that the with-context can work with any special keyword argument, but i left in the pushd alias because it will probably be used the most:

from sh import args, pwd, pushd

print pwd()
with args(_cwd="/tmp"): # or pushd("/tmp")
    pwd()
print pwd()