Added support of maven <plugin> in pom.xml (pom-plugin tag)

_Changes_
added :pom-plugins tag to project.clj
added example to sample.project.clj

_Motivation_
1. plugins section is commonly used in maven. 
2. lein pom should generate pom.xml with <plugins> section
3. tag :extensions has similar behavior
4. using pom-addition is inappropriate in this case as it generates code outside of <build> section.