allow shell to be overridden

[markbirbeck]

It should be possible to use different shells to run commands, such as PowerShell on Windows.

[markbirbeck]

@mvoidex has proposed a nice solution at pull request #32. However, looking at it a bit more I think it's worth breaking the whole problem down a bit.

The solution provided by @mvoidex uses variable substitution into the command to allow extra characters to be added. However, Popen() already provides for an executable argument, so instead I'm going to allow that to be configured -- as step 1 -- and then provide another mechanism to add in the extra characters to the command.