typo: REVEAL_MATH_JAX_URL -> REVEAL_MATHJAX_URL
Add missing end quote for source blocks

Fixes #53.
Expand full path when opening exported HTML file

Sometimes path is relative and browser fails to open the file.
Merge pull request #52 from aJchemist/patch-1

typo: REVEAL_MATH_JAX_URL -> REVEAL_MATHJAX_URL
Merge pull request #54 from lvh/patch-2

Add missing end quote for source blocks
Merge pull request #55 from maio/patch-1

Expand full path when opening exported HTML file