Adding support for space separated sexagesimal coordinates in SkyCoord

It came up on gitter that 
```
coordinates.SkyCoord('19 23 40.001395 +14 31 01.550347', unit=(u.hour,u.deg), frame='icrs') 
```
doesn't work, while it did with ``coordinates.ICRS``, and it would be useful to have this feature back. 

cc: @eteq, @keflavich