Add options and writable to exec, shell, run

[vsoch]

This pull request will allow the user to specify a list of options to each of exec, shell, and run, and also if the execution should be --writable. I've also updated the documentation (and docstrings) with these examples.

This will close #138

[vsoch]

@JayjeetAtGithub you can test this fix. First, remove all previous spython versions. Do this until it tells you it is installed:

pip uninstall spython

Then, clone this branch and install:

git clone -b add/options-writabl  https://github.com/singularityhub/singularity-cli
cd singularity-cli
python setup.py install

And then try the commands that were not working for you! I've updated the docs here to show them as well.

[JayjeetAtGithub]

@vsoch Thanks a lot! All well now.

[vsoch]

Ready to merge?

[JayjeetAtGithub]

Yep 😄

[vsoch]

You got it! Please give me a few minutes to release to pypi, I will post the link here when it's done.

[vsoch]

https://pypi.org/project/spython/0.0.72/