Change the quick test script to use a doc object

This allows using markdownlint's doc object methods as used in rules when
troubleshooting a document.
Move view_markdown to a tools dir and add another quick test script
Encode all text as ascii before passing to kramdown

Fixes #52

This works around some issues with unicode characters causing issues with line
numbering, and fixes a crash bug in markdownlint.

While no current rules rely on non-ascii characters, this may not always be
the case (e.g a rule that checks for the use of smart quotes in text), and so
this workaround should be considered temporary until a fix is found to
https://github.com/gettalong/kramdown/issues/158