Better descriptions for Polar Charts keys

- improves desc. for 'r', 't' in Scatter, Bar & Area
- improves desc. for 'direction' and 'orientation' in Layout
- adds details to desc. of 'range' in AngularAxis
- 'domain' in AngualarAxis added back (but does not work)
- showline now generated with make_showline()
- adds 'number_array' val type (for 'r')

And a few small fixes

- required_when is not a function anymore (not in val_types)
- correct 'orientation' val type (now "'v' | 'h'")
- fix typo in 'scl' example (now 'Greys')
- fix typo in 'domain' example (now [0,0.4], [0.6,1])
fix small typo in 'orientation' @ 'type'
adds href example to 'text' in 'annotation'
Clean up TODO comments
fix typo in 't' in Area
better handling of conditionally required keys
add closing ) in 'exponentformat'
better desc. for 'domain', list of all HTML tags in Annotation
Merge pull request #13 from plotly/polar-chart-keys

Better descriptions for Polar Charts keys