Create standard date/datetime/duration types

In the same way we provide `tags_type`, `get_enum_type`, `get_three_state_flag` and so on, we should offer a standard fixture for receiving user-friendly date and datetime input formats and converting them to ISO8601 that most services use on the wire.

These fixtures should:
- [ ] Recognize and accept ISO8601 format
- [ ] Detect user-friendly format and convert to ISO8601.

---

### Environment summary

Install Method (e.g. pip, interactive script, apt-get, Docker, MSI, edge build) / CLI version (`az --version`) / OS version / Shell Type (e.g. bash, cmd.exe, Bash on Windows)

```
azure-cli (2.0.27)

acr (2.0.21)
acs (2.0.26)
advisor (0.1.2)
appservice (0.1.26)
backup (1.0.6)
batch (3.1.10)
batchai (0.1.5)
billing (0.1.7)
cdn (0.0.13)
cloud (2.0.12)
cognitiveservices (0.1.10)
configure (2.0.14)
consumption (0.2.1)
container (0.1.18)
core (2.0.27)
cosmosdb (0.1.19)
dev-tools (0.1.1)
dla (0.0.18)
dls (0.0.19)
eventgrid (0.1.10)
extension (0.0.9)
feedback (2.1.0)
find (0.2.8)
interactive (0.3.16)
iot (0.1.17)
keyvault (2.0.18)
lab (0.0.17)
monitor (0.1.2)
network (2.0.23)
nspkg (3.0.1)
profile (2.0.19)
rdbms (0.0.12)
redis (0.2.11)
reservations (0.1.1)
resource (2.0.23)
role (2.0.19)
servicefabric (0.0.10)
sql (2.0.21)
storage (2.0.25)
testsdk (0.1.0)
vm (2.0.26)

Extensions:
subscription (0.1.0)

Python location 'C:\Users\trpresco\Documents\github\azure-cli\env\Scripts\python.exe'
Extensions directory 'C:\Users\trpresco\.azure\cliextensions'

Python (Windows) 3.6.1 (v3.6.1:69c0db5, Mar 21 2017, 17:54:52) [MSC v.1900 32 bit (Intel)]
```