Allow overriding session duration

[thepatrick]

• Adds --aws-session-duration argument
• When set to > 0, will be used instead of the value found in the parsed SAML (and where that is not set, the default value)

Should solve #13

TODO

• [ ] Tests
• [ ] Daemonizer

[ruimarinho]

@thepatrick I'm making some improvements to this PR draft :) will try to push changes later today.

[ruimarinho]

Thank you for the initial draft @thepatrick. Landed on https://github.com/ruimarinho/gsts/commit/0e3c49d1bdc33f204a5e1e34c3aaf8d54a664648.