Create VSIXs for each targeted OS/distro for offline installation

[DustinCampbell]

We need separate VSIXs that target each OS/distro that we support for the purpose of offline installation:

• Windows
• OS X
• Linux (Ubuntu, Debian, RHEL, CentOS)

These will include the appropriate debugger and OmniSharp bits so that they don't need to be downloaded.

[DustinCampbell]

@chuckries, I just heard you're looking at this.

[chuckries]

@DustinCampbell yes, I am about to submit a PR that adds a gulp task to do the debuggers installation up front.