Slightly changed the todo gem

Hi!

I needed some additional functionality on your todo gem, like a identifier for each task (task_id) which allowed me to discuss (online) about one or the other task with others, without always writing the whole description. Furthermore I needed a date and time of when a task was created (created_at). 

Additionally I changed the output of 'todo list', by showing the new fields, and added some color (yellow) to the output, or (red) in case of an error with some command. 

The 'todo remove' now accepts only an integer for the task number (index), I thought no one will ever delete by reinserting the description.. :-) 

I also removed the 'todo create' since I thought a 'todo add myTask' should create the file if it does not exist. 

And I changed from '.todo/list.yml' to simply '.todo.yml'. I couldn't see any reason why having a file in a hidden dir instead of just a hidden file. 

Last but not least, some specs.

But, I tried to keep it very minimalistic as you did it. I like it this way!