Allow strings as skill levels

[VictorBargains]

Currently, the description property of a wp_resume_skill term is used to store a numeric value which is then passed as a width to the span.skill-level with role="progressbar" set.

It should be possible to use a short string as this value, such as "Beginner", "Intermediate", etc. If the description has length but is not a number, it should be given width of 100% and that text should be displayed inside.

The WP_Resume::skill_box() function should be updated so that spaces get replaced by dashes when constructing the class name "skill-level-<?php echo $desc; ?>"