Allow extensions to support commandline arguments.

[ericsnowcurrently]

Currently I can start VS Code from my terminal by running code path/to/file. The code command supports a number of commandline options as well. However, neither those options nor the CLI "paths" appear to be open to extension by extensions. In other words, some extensions may benefit from exposing some of their capability at the code commandline, whether through additional CLI options and support for extra paths.

Here are some examples of how this might be useful:

• (see Suggestion: Start VSCode debugger from the command-line #10979) an extension could add options to support launching the debugger for a given file (or debug config), e.g. code --launch "my-config" path/to/file or `code --attach "my-config" .'
• (see Support opening a remote file or folder from the VS Code commandline. vscode-remote-release#1372) the remote-ssh extension could open a remote file/folder from the commandline, e.g. code ssh://my-host/path/to/file or code --ssh my-host path/to/file

[sandy081]

Closing this since this is being tracked in remote - microsoft/vscode-remote-release#585 and it is the main use case driving it.