Config API updates and improved go docs

[jentfoo]

These changes are designed to improve consistency and simplicity of chart configuration.

General Changes

• Public config struct fields have had their docs updated and improved.
• Type has been renamed to OutputFormat, this better describes the format that you want the chart rendered to.
• Heavier theme use - color specifications that were already possible to derive from the theme have been removed, instead the theme is now just always used. As a future change we will allow easier theme modifications to more easily allow one off customization.
• TrueFlag() and FlaseFlag() have been renamed to just True() and False()
• NewFloatPoint(float64) has been renamed to FloatPointer
• Fields with Option in their name have had the Option removed. This makes names more concise, and for YAxisOptions renaming to YAxis made it more consistent when compared to XAxis.

Axis-Specific Changes

• YAxisOptions field renamed to YAxis as mentioned above.
• FirstAxis has been renamed to the more descriptive DataStartIndex
• StrokeColor and SplitLineColor has been removed (as mentioned above, theme is now used). You can use WithAxisColor(Color) to easily change an existing theme to a different axis color. We plan to improve this API further.
• YAxis Color has been renamed to the more descriptive AxisColor. Depending on how easy we can make theme adjustments this field may be deprecated. It is preferred to modify the theme using WithAxisColor(Color).

Chart-Specific Changes

• Line charts Opacity has been renamed to the more descriptive FillOpacity (avoid confusion with other shading opacity)
• For Grid and Table charts Column and Row were made plural into Columns and Rows
• For Table charts Padding was renamed to the more descriptive CellPadding (avoid confusion with chart padding).