buttonize filter needs work & clashes with external links filter

[mDuo13]

• should be possible to customize the classes it adds
  • should probably default to Bootstrap btn btn-primary
• currently the example page doesn't include the style for the default class button (would be fixed if we switched it to bootstrap though)
• if you use with the external links filter and you have a link that is both external and a button, one of them tends to clobber the other. They should play nice.