Add partial support for Erlang/OTP 17

Allow rebar to compile applications using Erlang/OTP 17 and older
versions.  This patch only provides partial support since the rebar
tool itself must be compiled using an Erlang/OTP version that is older
than 17.
Add {prefix, atom()} option for abnfc_opts

Specify the atom to use as a prefix for the decoder and generator
functions inside the parser's module.  Default is ''.
Add {parser, module()} option for abnfc_opts

Specify the module to use for parsing the ABNF grammars.  Default is
'abnfc_rfc4234'.
Add {qc, proplist()} option for abnfc_opts

Specify the options to use for the generator functions inside the
parser's module.  Default is [].