adding texgrep

Small tool to (e)grep in TEX files.
Works fine in cooperation with `bibgrep` as that one puts the found
BIB key in the X clipboard and `texgrep` (if no parameters were given)
uses that clipboard as search term:

    $ bibgrep Kato
    ../../bib/linux.bib:120
    @inproceedings{Kato2010Airs,
      [...]
    }
    ../../bib/linux.bib:344
    @inproceedings{Kato2008Modular,
      [...]
    }
    $ bibgrep Kato2010
    ../../bib/linux.bib:120
    @inproceedings{Kato2010Airs,
      [...]
    }
    $ texgrep
    TERM='Kato2010Airs'
    chap01b_related.tex:366:\citetitle{Kato2010Airs}