Filter hooks for <meta> elements

[justintadlock]

For <meta> and other elements in inc/functions-head.php, apply filters to allow theme authors to more easily change these.

Needed for the following functions:

• hybrid_meta_charset()
• hybrid_meta_viewport()
• hybrid_link_pingback()

[justintadlock]

We have filter hooks for these on the 5.0 branch.