Add filter log by "today"

[dhontecillas]

In the docs says that we can pass a today param to the ti log command, but it is not implemented.

I tried to add the functionality using a Period class to check for intersections of time. I though that in the future we can improve and add more time periods, like "this week" , or "before 2018-01-01", and so one.

I did not add unit test, because there is no file for tests yet, but I can add them if you are ok with it.