Fix cover print truncation when coverage is 100%

When the option {cover_print_enabled, true} is set and that the code
coverage is 100%, the '%' will end up being truncated.

This patch makes it so that the truncation happens at 4 a characters
width so that instead of the following printouts:

    some_mod  : 99%
    some_other: 100

We instead show:

    some_mod  :  99%
    some_other: 100%

This solves issue #331 as reported by @lispking and @NineFX.
Merge pull request #371 from ferd/cover-print-percent

Fix cover print truncation when coverage is 100%
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 [].