add caption for image
can't press enter into caption
Merge remote-tracking branch 'origin/feature/add_caption_for_image' into feature/add_caption_for_image

Conflicts:
	dist/js/addons/medium-editor-insert-images.js
	dist/js/addons/medium-editor-insert-images.min.js
	dist/js/addons/medium-editor-insert-plugin.min.js
	dist/js/medium-editor-insert-plugin.all.js
	dist/js/medium-editor-insert-plugin.all.min.js
	src/js/medium-editor-insert-images.js
modified caption dom
can't press enter inside caption
embeds tag into placeholder