TexCount can parse included files

Now, the word count will include details about any files that are
included in the selected file. Also, additional options can be
passed to TexCount using the $TM_TEXCOUNT_OPTIONS variable.
Do not remove first line of output in “TeXCount”

The old behaviour could lead to problems if there was only one line of
output. This could happen if we for example set `TM_TEXCOUNT_OPTIONS` to
`-brief -merge`.
Improve output of “TeXCount”

We now directly provide the filename for `texcount`. Directly
providing the absolute path to the file leads to very long lines in the
output of texcount. Therefore we now also change the working directory
to the one where the tex file resides before we call the command.