Add `--read-logs-from` for asynchronous log output

[9999years]

Because ghciwatch reads ghci's output and expects it to be structured in a standard way, it can't cope with programs that output to stdout (or stderr) asynchronously (see https://github.com/ndmitchell/ghcid/issues/137).

To make this easier for frameworks that output logs (like Yesod), we should have an option to read logs asynchronously from a file. If provided, writes to the given file will show up in ghciwatch's output (but not ghci's output).