Introduce URL field type:

* New field class extending `BP_XProfile_Field_Type` for URLs
* Clean-up existing `BP_XProfile_Field_Type` extensions (formatting, `bp_parse_args()` VS `array_merge()`, whitespace, brackets, etc...)
* More to do here, specifically regarding output and `display_filter()` methodology.

Props boonebgorges, williamsba1, sooskriszta. See #5501, #5630.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8632 cdf35c40-ae34-48e0-9cc9-0c9da1808c22