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.