use sidebar name rather than hardcoded labels

In general, this is a good idea so that you load the theme's name of the sidebar (localized in the theme) rather than someone having to localize this plugin as well.

Another benefit (specific to me) is that I can run my <a href="https://github.com/billerickson/genesis-left-right-sidebar">Genesis Left Right Sidebar</a> plugin alongside this plugin without a hacky gettext translation.