Skip check for ebin directory when building edoc

edoc target fails if ebin directory does not exist.
ebin directory is not necessary for building edoc.
Skip check that ebin directory exist.
Merge pull request #376 from irpab/edoc_without_ebin

Remove check adding ebin to path for edoc target
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 [].