Add VocabTextLexiconParser for simple text-based lexica

[larissakl]

Support for simple text lexica containing the vocabulary/list of lemmas.
This is can for example be used in the Lexiconfree beam search (#101) for which the XML-format is unnecessarily complicated.
Instead of an XML-lexicon, one can use a txt-file containing one label per line.
For example

A
B
C

will be parsed to a Bliss Lexicon equivalent to

<?xml version="1.0" ?>
<lexicon>
  <phoneme-inventory>
    <phoneme>
      <symbol>A</symbol>
      <variation>none</variation>
    </phoneme>
    <phoneme>
      <symbol>B</symbol>
      <variation>none</variation>
    </phoneme>
    <phoneme>
      <symbol>C</symbol>
      <variation>none</variation>
    </phoneme>
    <phoneme>
 </phoneme-inventory>
  <lemma>
    <orth>A</orth>
    <phon>A</phon>
  </lemma>
  <lemma>
    <orth>B</orth>
    <phon>B</phon>
  </lemma>
  <lemma>
    <orth>C</orth>
    <phon>C</phon>
  </lemma>
</lexicon>