textutils: translate LaTeX symbols to Unicode

• Adds a function in textutils that replaces LaTeX symbols to Unicode using an appropriate translation table.

• Adds a default translation table mapping common LaTeX symbols to Unicode. Adds external contributions in THANKS.

• Adds function decode_to_unicode() which attempts to detect input character encoding and decode it into Unicode. For optimal results it is recommended to install the 'chardet' module.

• Also adds appropriate regression tests.