Allow to use the flag 'a' in the transformation commands.

With the current behavior of ultisnips and the tex snippet, we lost accentuated characters in the labels of the sections.
For example:
\section{Cédille ç}
\label{sec:c_dille_}

This patch allows to transform the text in ASCII and apply the transformation on the ASCII version (i.e., accents are removed from the letter), which then allows to have the letters in the labels.
For example:

\section{Cédille ç}
\label{sec:cedille_c}


To work with an asciiversion, it is necessary to use a new flag: 'a'
The patch works if the unidecode package is installed