Add an html-pipeline executable to the gem

Usage: html-pipeline [-h|--help|help] [-f|--filters]
       html-pipeline [FILTER [FILTER [...]]] < file.md
       cat file.md | html-pipeline [FILTER [FILTER [...]]]

Also stop gitignoring bin/, but ignore exec/ instead for binstubs.
Merge pull request #44 from indirect/master

Add an html-pipeline executable to the gem