Add an option to check for updates on startup

[fretep]

Added an option to disable checking for updates so if todotxt.net is unable to access the Internet, the user can disable update checking on startup to prevent constantly having "New version" showing in the menu.

[fretep]

Sorry! This is my first time using Git so I'm learning. I obviously was too focused on getting Git installed and figuring out Git, and not focused enough on the quality of the content. I see I now have a conflict to resolve, so I'll work through that and your suggestions in the next few days.

This is a bit odd though. I'm not questioning your authority or correctness here, you clearly know your code better than me, and the files you have listed when I look at them seem like I definitely need to make changes to them, however, I am questioning how I missed these files.

When I run this pull request on a clean system with no prior settings, the option is both defaulted to enabled, honoring the setting even on first run and persisted in the configuration across runs. I did test those scenarios before submitting the pull request. I am running the portable version, and did not check the non portable code, so perhaps that is the difference. I'll dig into this and figure out the piece of the puzzle I'm missing before I come back.

Thanks Ben.

[fretep]

Updated to address issues. Hopefully I got it right this time.

[benrhughes]

Looks good, thanks