Fixed whitespace management in section names

Hi, if you provide a section name with whitespaces inside, this result in having a filename with spaces in the 'contents' folder, which doesn't seem to work with pdflatex (at least in the current version of pdflatex from texlive as provided by the ArchLinux repos).

Example:
[marcello@laptop-arch rlatex]$ ruby rlatex.rb new prova --sections "Sezione con tanti spazi/"Primo Paragrafo", AnotherOne "Ancora degli spazi nel titolo"/Sottosezione --author "A Random Name" --language italian

With this fix, all whitespaces in the section name provided by command line are converted to underscores, and then managed accordingly by the existing code.

Cheers,
Marcello