Customizer: Support `textarea` and commonly-used input types as control type in `WP_Customize_Control`.
Add `input_attrs` property to support custom input attributes.

(Demo plugin attached to ticket.)

props celloexpressions.
fixes #28477.
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git-svn-id: http://core.svn.wordpress.org/trunk@28728 1a063a9b-81f0-0310-95a4-ce76da25c4cd