Suggested updates to QMarkdown

[hawkeye64]

Note: This is a breaking change for the 2.x-alpha

It's been suggested that QMarkdown not contain some of the markdown-it plugins that it currently has. For instance, the emoji plugin is large and carries a lot of weight with it. Instead, this (and others not common) would be removed and in it's place, an exported function would be provided where you can add global plugins yourself (like the emoji plugin) if you need it. This alleviates a size and speed in parsing issue and allows convenience from the global aspect.

Please comment below if you have concerns.