Customize the extensions for markdown processing

Hi Eric, 

I took the liberty to add an argument `pegdownExtensions` that allows specifying the extensions to use when processing markdown with pegdown, specifically because I hate the HARDWRAPS extension with all my heart. =^.^= 
I updated the user guide accordingly but added no new tests. The general argument tests itself seemed to already cover a lot of ground.

I hope you find the addition acceptable.

Kind regards
Andreas