Allow choice to not display spinner while waiting for process to terminate

[djmattyg007]

Displaying a spinner to the user is not always desirable, such as when
code is being called as part of an automatic script rather than purely
for a command line application. This lets the user choose whether or not
the spinner should be displayed.

[coveralls]

Coverage decreased (-0.3%) to 91.508% when pulling 503ae7d on djmattyg007:improve_spinner_control into 856da2f on xolox:dev.

[xolox]

Hi Matthew and thanks for the pull request! I've merged your changes but made some modifications in the process, specifically executor now uses Spinner(interactive=False) when use_spinner is False, this disables the rendering of the spinner without requiring an if/else construct in wait_for_process() to avoid the spinner's with block.

[djmattyg007]

Awesome :) Thanks, and thank you for all of your python packages, they're all superb.