Add option to parse imports only and an option to pass file contents

[ramya-rao-a]

• Autocomplete feature in VS Code Go extension uses go-outline to get imported packages. Having a "imports-only" option will help in skipping the overhead of parsing the complete file
• Autocomplete feature in VS Code Go extension also needs to run when the file has changes that is not saved. Therefore, an option to pass the file contents to go-outline would help too

[lukehoban]

Looks good!