teximg: Use Unicode UTF-8 encoding by default.

teximg: Use Unicode UTF-8 encoding by default.

If I am not mistaking all source files in ikiwiki are encoded in Unicode UTF-8.
Adding `\usepackage[utf8]{inputenc}` enables LaTeX to deal with the encoding.
As a consequence some special characters like umlauts can be used in the source
code which is useful for foreign languages.

        [[!teximg code="a = b \text{ für alle } b \neq 2"]]

But for example »≠« cannot be used in LaTeX right now. One has to use other TeX
systems like XeTeX or LuaTeX featuring native UTF-8 support or use additional
nonstandard packages like uniinput [1].

I used the package `inputenc` (`texdoc inputenc`) and not `inputenx` (`texdoc
inputenx`), because I have not used `inputenx` that much and using the option
`math` is not supported in Debian (and I guess other distributions too) since
`inpmath` is not included in CTAN.

[1] http://wiki.neo-layout.org/browser/latex/Standard-LaTeX

Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>