self/child selection for placeholder

Placeholder styles can define for element itself or child elements
Arrow mixin applies only for single :pseudo

Rather than using both :before and :after, arrow mixin uses only one of
the if single colour and single size defined. In two color and size
defined, it uses the classic approach
New tests

Tests html filed renamed
Arrow and placeholder tests added
CSS3 mixing comments
Tests updated and version file added
String functions initial
v. 0.2.0