fix: restore auto indent size in markup editor

[smeijer]

What:
#128 changed the behavior of attribute indentations in the markup editor. This PR restores the old behavior.

Why:
Because it looks better. Compare:

<div 
     class="container">
</div>
<textarea
          class="input">
</textarea>

with:

<div
  class="container">
</div>
<textarea
  class="input">
</textarea>

How:
Pass the multilineTagIndentPastTag: false option to CodeMirror

Checklist:

• Tests
• Ready to be merged