Format.prototype.removeConflicting removes all formats than cannot coexist

Line formats with 'exclusive: true' automatically remove all other
exclusive formats when applied.

The 'bullet' and 'list' formats are refactored to use 'exclusive: true'
Add support for h1, h2, h3
Use 'P' as the default block tag, fix some styles as a result
Merge branches 'default-block-tag' and 'headers'