Allow information about text length in teiHeader

[christofs]

In the keywords section, we should allow for something like the following:

<term type="length-divs">12</term>
<term type="length-lines">123</term>
<term type="length-words">1234</term>
<term type="length-chars">12345</term>

Or, maybe more TEI-conformant:

      <extent>
        <measure unit="div">divs</measure>
        <measure unit="lines">4201</measure>
        <measure unit="words">143824</measure>
        <measure unit="chars">838850</measure>
        <measure unit="kB">840</measure>
      </extent>