Allow registering OutputFormats without file extension

[moozzi]

No description provided.

[postmodern]

I realized later this will require changing how infer_from(path) works. Maybe it might be easier to have ext default to an empty String, since File.extname('foo') # => ''.

[moozzi]

But '' will be used for few different output formats..

[postmodern]

@AI-Mozi hmm yeah. We should keep register :dir, '', Dir that way ronin-recon run --output path/to/dir will select the OutputFormats::Dir output format, but Archive/GitArchive should not specify a file extension and would need to be specified explicitly (ex: --output-format archive or --output archive:path/to/dir).