CLI --extension flag should check for valid extension

[johntrandall]

Currently,
jsx -watch --extension jsx /scr /build
launches the JSX compiler CLI. However, it does not compile files with the .jsx extension as expected. The 'dot' is missing.

To make the CLI work correctly, the user needs to type (note the added 'dot')
jsx -watch --extension .jsx /scr /build

This seems to be a common user mistake. it would be nice if the CLI was more forgiving with it, or threw a warning.

I think the user can be forgiven for the mistake. It is natural to expect that an 'extension' flag receives only an extension itself, rather than an extension with a preceding dot. The Semantics are tricky here.

[johntrandall]

nv. I'm an idiot.