Safety/QOL fixes

[akx]

This PR makes transformations a little safer and easier to script.

• Safety: a temporary .tsx file is first created as the transformation target, instead of wholesale renaming the original JS file to .tsx for transformation; if transformation succeeds, the target .tsx is written and the original and temporary files are deleted (unless the new --keep-*-files flags are set). (Before this, the original .js file was renamed but left untransformed if there were problems.)
• QOL: A new --ignore-prettier-errors option was added, to make it possible to use "almost working" TS files (see Named-function SFCs get mistransformed #38)
• QOL/scripting: The CLI now exits, as is customary, with error code 1 if there were any transformation problems.
• QOL/bugfix: Multiple files/glob parameters didn't work correctly before; only the first argument was parsed, and since glob expansion occurs at shell level, one would have had to invoke the command using bin/... 'my/**.tsx' (note the extra quotes). Multiple parameters (i.e. shell expansion) now works (but Node-level globs do too).
• QOL: A progress message was added to the otherwise very quiet CLI. :)

Thanks for the project – I've had lots of fun, even, converting codebases to TypeScript :)

[mohsen1]

Thanks for your contribution! I'll review and provide more feedback. In the meantime I'll try to fix the issue you raised in #38. We should never print invalid TS and Prettier should never fail.