Specification of datetime ranges

Make specification of datetime ranges more flexible.
Currently only datetimes in ISO format or simple times are allowed:
```
timew track 2018-12-12T09:00 - 2018-12-12T11:00
timew track 20181212T0900 - 20181212T1100
timew track 09:00 - 11:00
timew track 9am - 11am
```
It should be possible to specifiy datetime ranges also as ` - `, e.g.
```
timew track 2018-12-12 09:00 - 11:00
timew track 20181212 9am - 11am
timew track 9 - 11 2018-12-12
timew track monday 9 - 11
timew track :yesterday 9am - 11am
```
The remaining constraint would be that the given times are in the same format, e.g.
```
timew track 2018-12-12 09:00 - 11am
timew track 2018-12-12 0900 - 11:00
```
would be invalid.