Support for Context Group Location

[mbeckenbach]

Hi,

first: This library is super cool. I'm using it to build a simple tool for translating my angular 4 app and it saved me a lot of pain. Thank you!

Angular's i18n extract tool generates a context-group which contains the source code location. Would it be possible to add this to the XlfTransUnit class?

<trans-unit id="toolbar_search_placeholder" datatype="html">
        <source>Search...</source>
        <context-group purpose="location">
          <context context-type="sourcefile">app\shared\toolbar-search\toolbar-search.component.ts</context>
          <context context-type="linenumber">12</context>
        </context-group>
        <target state="final">Suchen...</target>
      </trans-unit>