add 'update_tag' option

[iTrooz]

This PR does two things :

• adds the 'update_tag' option
• refactor the big try/catch block, to allow better error handling (a random error will not create a new release)

Fixes #171

[sithlord48]

I need this feature for continuous releases . @softprops How can I help test this ?

[mkurz]

@softprops Can you please take a look here finally? This is a very useful feature and our project would highly benefit from it. Thank you very much!