Add Sorting Functionality

[MichaelLCrispen]

Something that would be pretty cool to use with this program would be sorting the todos by closest date due / furthest date.

[aesophor]

Hi, I've implemented sorting in 5b1e355edae1df26b729200af32a1d0558541692

Here's an example usage:

$ todo                                               
You have 3 items left on the reminder!
0) ItemA (22 days left)
1) ItemB
2) ItemC (Next Tuesday; 4 days left)

$ todo -s                                            
You have 3 items left on the reminder!
0) ItemC (Next Tuesday; 4 days left)
1) ItemA (22 days left)
2) ItemB