Add self_update option to git_cm app

[CPerezz]

Added the feature mentioned in #30 .

Everything looks fine. We could add a specific clap::Arg in order to allow the user go for the update directly instead of notifying him every single time there's a new release out. But since I only check major release changes, the user won't probably get too many messages.

This is the output I get atm:

./target/debug/git-cm
? A newer version for this crate was found.
✔ A newer version for this crate was found.
Would you like to update your application? · yes
Checking target-arch... x86_64-unknown-linux-gnu
Checking current version... v0.0.1
Checking latest released version... v0.1.4
New release found! v0.0.1 --> v0.1.4
New release is *NOT* compatible
An error ocurred during the app update process: ReleaseError: No asset found for target: `x86_64-unknown-linux-gnu`

As you see, there's a problem, it comes from: jaemk/self_update#52 and we have two options. Even this issue gets fixed there (not a big deal I think since looks pretty easy to solve). Or we rather publish releases depending on the target where they'll be built for.

In any case, let me know your thoughts @sirwindfield 😄

[mainrs]

Thanks for the PR! I won't merge it right now though as I am still implementing stuff over at @conventional-commits-rs to allow for automatic version releases. Once I've setup that, I will merge this. Doesn't really make sense to include self update functionality if the only way to install it right now is through cargo itself :)

[CPerezz]

Makes complete sense!

Feel free to leave this here until it is useful to merge it!