Add --stdio to process STDIN

[leonbohn]

This adds the switch --stdio which will skip the provided list of files and instead process the text that is provided via stdin. The formatted output is then written to stdout.

The list of files is now no longer a required argument, but within the Cli::resolve method, it is ensured that either the --stdin flag is set, or at least one file is provided. Also, the method warns if a list of filenames is provided and the --stdio flag is set, and then clears the list of files and processes just stdin.

The logging is now initialized a bit earlier in the main() method, which allows writing a one-off error message (by invoking log::error!/log::warn!) within Cli::resolve.

[WGUNDERWOOD]

Thank you, this looks great! I will merge it and make a few small edits myself.