Shebang detection for Tcl/Tk scripts

Linguist currently does not detect properly extension-less Tcl/Tk scripts.
It resorts to the statistical classifier, which yields rather poor results.

See as examples:
* code from http://wiki.tcl.tk/906 detected as Perl (added to the test suite)
* code from http://wiki.tcl.tk/14140 detected as Shell (added to the test suite)
* laomaiweng/brc:brc detected as JavaScript
* laomaiweng/scripts:admin/gentoo/puse detected as Perl

Relying on the shebang enables correct detection of all shebang-ed Tcl/Tk
scripts.