Create a separate branch for VS2015 legacy support

[justcla]

So that EasyMotion can progress and use libraries from v15.x and beyond, I suggest we create a separate branch for a VS2015-only version.

I have created such a branch on my fork. justcla/EasyMotion/VS2015

Can you please create a branch on your main fork so I can push my branch to you?

Thanks!

[justcla]

Once VS2015 is settled into a new branch, I will upgrade the References in the project to point to v15.x+ versions.
Then the extension will only run on VS2017+.

I would recommend creating a new page on the Marketplace for EasyMotion-VS2015.vsix.

[jaredpar]

@justcla agree this is a good idea. In order to unblock you better I've given you write access to the repo. This way you're not blocked on me anymore to make progress here.

[justcla]

Thanks!