feat: multiple files to single output file

[tadhgmister]

Fixes #272

• adds new option --exportMainOnly (default to false) which causes all definition types to be not exported
• adds new option usedNames (not available to command line) which allows the same set of used names to persist over multiple calls to compile (this was main part needed to have single output file work correctly)
• when input is multiple files and output is stdout it now uses a continuous usedNames set so piping to a file works as expected
• when output is a single file and input is multiple files it writes it to the file similarly to outputting to stdout but it also overrides the banner for subsequent files to just be a separator.

[bcherny]

Closing out stale PRs. Feel free to rebase and re-open if you like.