Add ability to parse markdown task list to check for completion

[MatthewCroughan]

This may be a bit monolithic and contradictory to the nature of the program, which encourages piping into things. If there are features like this that are considered monolithic we should add an example section to the README which includes some great suggestions.

What I want to be able to do is search for all the instances of this in my notes:

- [x] Do other thing
- [ ] Do thing
- [x] Report X to Y 
- [ ] Get eggs

I then want to be able to filter out the incomplete notes and the inverse of that. I would also like an option to print the date of the note this came from. This way we can find incomplete tasks and print the date that they were supposed to be completed.

I can probably figure out a way to do this with grep and sed, so if I do we should add it to an example usage section for people to benefit from.