Themes: Allow template loading functions to pass additional arguments to the...

Themes: Allow template loading functions to pass additional arguments to the template via the `$args` parameter.

This affects:
* `get_header()`
* `get_footer()`
* `get_sidebar()`
* `get_template_part()`
* `locate_template()`
* `load_template()`

Note: `get_search_form()` already passes additional arguments to the template as of [44956].

Props enrico.sorcinelli, sc0ttkclark, scribu, nacin, wonderboymusic, GeertDD, beatpanda, amaschas, mintindeed, ysalame, caiocrcosta, bigdawggi, julianm, eddiemoya, shawnz, sayedwp, shamai, mboynes, mihai2u, guidobras, Mte90, apedog, stuffradio, overclokk, johnbillion, joyously, afercia, audrasjb, justlevine, SergeyBiryukov.
See #21676.
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git-svn-id: https://core.svn.wordpress.org/trunk@48139 1a063a9b-81f0-0310-95a4-ce76da25c4cd