add support for reading back file contents partially

While `cat` and `tail` and `grep` are useful for reading back portions of a data file, they're not cross-platform, and with some awareness of the data in the file, we could be a little more helpful. Some useful features could include:

- [ ] Return all items from a specific date, e.g. `tot read things-i-ate --date 2020-09-25`
- [ ] Return most recent items going back a specific amount of time `tot read exercise --last 4h`
- [ ] Return all entries within a specific time period `tot read exercise --from -12h --to -2h`
- [ ] Return all entries containing a specific value `tot read things-i-ate --where what:coffee`