Add support for "Github Flavored Markdown Task Lists"

[Hasan-75]

Github flavored markdown supports Task Lists.
It will be a good addition if the reversemarkdown-net can create task lists from a list of checkbox inputs.

Example:
HTML:

<ul>
  <li><input type="checkbox" checked> Complete the project report</li>
  <li><input type="checkbox"> Submit the report</li>
  <li>This is a regular list item</li>
</ul>

Markdown:

- [x] Complete the project report
- [ ] Submit the report
- This is a regular list item

Markdown Preview:

• Complete the project report
• Submit the report
• This is a regular list item

[mysticmind]

Closed via #405