Add a Mod field to ProblemDescription.

Hello guys, I wonder if this is a good feature to consider.

You know the counting problems that eask you to "return answer modulo 1,000,000,007", or sometimes 1,000,000,009 or something. Well, this features tries to detect that pattern, so you can have the following in template:
<pre>
${<if Problem.Description.Mod}
    static const int MOD = ${Problem.Description.Mod}; 
${<end}
</pre>

And it automatically adds it. As long as it recognized the pattern correctly. There may be problems in which it doesn't work for some reason, but so far tests work well.