Add option for custom command launch

[euclaise]

Instead of only offering the default shell, it would be useful to have a flag that allows for the launch of a custom program without having to go through the shell first.

For example:

Current behavior:

$ termtosvg myfile.svg
Recording started, enter "exit" command or Control-D to end
$ mycommand
 # interact with mycommand
$ exit
Recording ended, SVG animation is myfile.svg

Requested behavior:

$ termtosvg myfile.svg -c mycommand
 # interact with mycommand
Recording ended, SVG animation is myfile.svg

[nbedos]

I forgot about this, thanks for reminding me. Yes, that would help for #20 as described here and also for #11.

[nbedos]

There is an open pull request for this feature here: #84
Suggestions, reviews and testing welcome.

[nbedos]

This feature is included in the current release candidate which can be downloaded from test.pypi.org:

pip install --extra-index-url https://test.pypi.org/simple/ 'termtosvg>=0.7.0rc0'

[nbedos]

Feature included in version 0.7.0.