Adds an option to the theme customizer (inc/theme-customizer.php) and
corresponding changes to functions.php that allows a person using a
child theme to choose to pull Bootstrap (css and js) from the child
theme's css and js directories.
This change allows people using the-bootstrap to compile a custom
version of Bootstrap css files with LESS without overwriting the parent
theme's css files and avoid overriding default colors, fonts, etc. in
their child theme's styles. This can significant simplify child theme
customization and reduce the total size of CSS to load. Tools like
bootswatchr.com make custom Bootstrap easy even for people who aren't
comfortable with LESS tools.
This change also allows theme users to update Bootstrap out of sync with
the parent theme without overwriting the parent's copies of Bootstrap
files. This makes it easier to identify if problems are caused by
Bootstrap customization (now isolated to the child) or are in
the-bootstrap without custom Bootstrap files.