Updated "subscript" toolbar button markup

it was mistakenly opening a <sub> but closing it with </sup>.
Merge pull request #171 from litera/patch-1

Updated "subscript" toolbar button markup
Updates dist and src files
Do not show anchor preview for empty of # links, issue #167
Look for single quoted href attributes
Merge pull request #174 from yacomink/master

Don't show anchor preview for empty of # hrefs
Updates dist files
new option to support double return
Merge branch 'double-return' of https://github.com/zzjin/medium-editor into zzjin-double-return
Fixes default options spec, changes disabeReturn verification order
README typos
1.7.0
Prevents new lines with shift+enter when disableReturn is set to true
Removes duplication from test files
Removes DOM elements created by MediumEditor on deactivate fixing #164
Formatting
Fixes package.json
Adds extra missing fields to package.json
1.7.1
Changes activate to init the toolbar