Incommand

Added in_cmd function to issue a command to a single virtualenv.

### in ###

Run a command in the given virtualenv.

`usage: pew-in [env] [command]`

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Useful if you want to do things like:
`pew in [env] pip install -r requirements.txt`
after the initial virtualenv creation (i.e. in a shell script).

Also documenting the (undocumented) `inall` command and the new `in` command in the README.