Markdown amends text in multiple locations + other small issues with it

[wadeack]

Steps to reproduce:

1. create a note with some repeating lines
2. place cursor at the start of one of the lines from step 1
3. click on markdown

Expected result: Markdown only applied to current line

Actual result: Markdown is applied to every matching line

Other small issues:

1. Applying a markdown within the note (not at the end) adds markdown element to the bottom
2. Checkbox is inconsistent with other types as it does not require square brackets to be at the start of the line