Merge pull request #7 from marksteve/master

Minor improvements
Remove unused err variable
Add link to previewdocs
Merge pull request #11 from fgrehm/remove-unused-var

Remove unused err variable
Merge pull request #12 from fgrehm/add-link-to-previewdocs

Add link to previewdocs
Implement relative links fixing (#5)
Fixes and improvements to relative link fixing
Turns out url.IsAbs(/path) is false
Make md file extension optional
Update md exts
Updates based on @progrium's comments