GitHub task list items parse when the non-optional space after the [ ] or [x] is left out

[vsch]

The following should not parse as a task list item:

- [ ]this is not a task item

• [ ]this is not a task item

Instead it would parse as a task list item with [ ] or [x] following a task list item as in:

• [ ]this is not a task item

I have a fix but working on tests and I am in the middle of a big release for my plugin so this might take a bit. Anyone needing some fixes before they are merged into official branch can grab them from the develop branch of my fork: https://github.com/vsch/pegdown/tree/develop

I update the CHANGELOG to keep track of modifications that are made.