Proposal: new Check - MultiFileRegexpHeader

Rational:
in case there is company merging, then new copyright might be applied meanwhile the old files should keep the previous header.

Properties:
charset (like all other FileSetCheck having.)
fileExtensions (like all other FileSetCheck having.)
headerFiles, type:URI - list of files with header templates. As it isnot a lit of files and can be URL, URI should not have , in it, as it hard delimeter for list.

  <module name="MultiFileRegexpHeader">
    <property name="fileExtensions" value="java" />
    <property name="headerFiles" value="${config_loc}/java.header.01, ${config_loc}/java.header.02" />
  </module>

Header should match to any of defined header templates.