Add --stdin support

[zimbatm]

Mostly needed for editor integration.

When prjfmt is executed with the --stdin argument, it will read the content from stdin.

This is a different running mode. It needs to check that only one file path is being passed as an argument, or fail with a relevant error message. This is only used to check the format of the stdin file.

A new running function needs to be added, that reads all the content from stdin and writes it in a tmpfile. Ideally the tmpfile has the same extension as the passed-in path. Then use the standard formatter interface to format that file. Once finished, read back the temporary file and pipe it to stdout.

Depends on #7 to avoid posting formatter outputs on stdout.

[zimbatm]

Fixed by #95