Add a Markdown formatting toolbar

[GAZ082]

Would be handy to at least have the option to have a MD toolbar with the most common formatting options. If it's customizable would be even better.

Originally posted by @mbarczak in #2377 (comment)

[hokanovic]

the lack of a markdown toolbar is a dealbreaker for this awesome application

[arcturus140]

I disagree.

[colineberly]

The key to this is to have in non-intrusive. Like a toggle or a single button that shows a secondary menu. Ideally, a view menu toggle would be best, so that for the people that are well versed / dev / or like to write it all out manually, they can hide the toolbar and those who wish to use it can have it visible.

I don't think something like the way they've implemented it in Bear is the right way to go. Although it's visually "pretty", their implementation has the user make two clicks to get their formatting, plus the location is rather annoying (bottom right corner). Quiver on the other hand has a less intrusive toolbar.

It could be something as simple as this (crude example):