Add header field plugin for base_url

Here's patch for this.

Added some corrections in patch.

Thanks or the patch. The patch looks great. Also tested it in local, and it worked fine. No PHP warnings, etc.

Some improvements:
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+++ b/includes/views/handlers/views_base_url_handler_area_base_url.inc
@@ -0,0 +1,175 @@
+ * A handler to output site's base url.

Please update this to "Views area handler to output site's base url"

It would be great if we also add this option Use replacement tokens from the first row, but that is not a problem, we can add it later.

Needs work:
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Can you please also add tests.

Thanks :)