Add a new config value to toggle the parallel parsing

Is your feature request related to a problem? Please describe.

I was planning on running hank on Klarna's codebase, and one issue I found is that hank dies when trying to parse all the files we have in parallel.

Describe the solution you'd like

After talking with @elbrujohalcon, the proposed solution would be to have a new configuration value added to hank that enables/disables this parallel parsing, e.g.: {parallel_parsing, boolean()}.

When this option is set to true, it would change this line

rebar3_hank/src/hank.erl

Line 62 in 1c4b71a

rpc:pmap({?MODULE, get_ast}, [], Files).

to

lists:map(fun get_ast/1, Files).