Allow threshold of too long lines to be configured

Currently the plugin will only reject the commit if 33% or more of the
commit message's lines are longer than the limit.

Add a configuration parameter to allow the administrator to set this
threshold.  If not specified, it defaults to 33, the same as it is now.

Change-Id: Ic1f07ba1f27a7837a59d215ca943663cb85a12ff