Feat: Add ability to compile from stdin

[vanceism7]

This PR adds the ability to compile from stdin.

This is useful so that cppfront can compile source code that's been piped in from another program. For my use-case specifically: when working on the LSP server, we already have access to source code within a TextDocument variable.

Prior to this PR, we have to either save the edited content to a temporary file, or save the source file itself prior to invoking cppfront, otherwise we end up with stale diagnostics data.

Happy to fix any issues anyone finds!

Thanks!