Allow users to specify a list of files to process, non recursively

[elbrujohalcon]

We have --recursive in the command line, and after #12 we'll also infer files_r if they're not provided there.
But… so far the user can't specify a list of files (i.e. not folders) to type.
The proposal is then…

• Add --files option to rebar3 typer, that should accept a comma-separated list of files.
• Add files option to rebar.config's typer section.
• Parse these 2 and don't infer the src folders if they're set.
• Make sure that using --files combined with --recursive works in our plugin as it works in typer.
• Add files to `rebar3_mini_typer:opts()``
• Use files to populate #analysis.files in rebar3_mini_typer.