Allow users to add their own VSIX repositories to list script packages

[johnmcbride]

A new settings option should be added to the extension to enable the users to add multiple link locations where script vsix files are listed.
These could be

• RSS feed (that lists the vsix files)
• github releases page
• Fileshare?

The settings should be a multi-value setting
Setting should be set through the standard VSCode settings options.
List of VSIX packages should be listed in some type of UI from within VSCode.

[johnmcbride]

implemented in the latest release. We have implemented a setting that allows a developer to add an RSS feed as s source. This can be a FILE or HTTP based rss feed. We have also created helper utilities to create the RSS feed. Check out the tool at https://github.com/johnmcbride/citrix-script-feed-gen