[Library] Allow stat var <> color mapping to be set by users

[juliawu]

Adds an optional "colors" attribute to each of the chart web components, to allow the user to set specific colors for the stat vars being plotted. Any CSS valid color value is allowed.

Changes made:

• colors attribute is added to each chart web component
• the corresponding tile gets color added as a chart option
• some cleanup of attributes/props -- resorted to alphabetical order
• Examples and docs are updated

Examples:

<datacommons-bar
  title="Median income by gender of select US states (grouped)"
  comparisonVariables="Median_Income_Person_15OrMoreYears_Male_WithIncome Median_Income_Person_15OrMoreYears_Female_WithIncome"
  comparisonPlaces="geoId/01 geoId/02 geoId/04 geoId/05"
  colors="#4287f5 #f542a4"
></datacommons-bar>

For the maps, providing up to three colors are supported (will ignore colors past the first three):

For a single color, we get a luminance based color scale

<datacommons-map
  title="Population"
  place="country/USA"
  childPlaceType="State"
  variable="Count_Person"
  colors="red"
></datacommons-map>

For two colors, we get a diverging color scale

<datacommons-map
  title="Population"
  place="country/USA"
  childPlaceType="State"
  variable="Count_Person"
  colors="blue red"
></datacommons-map>

For three colors, we have [min, mean, max] mapping:

<datacommons-map
  title="Population"
  place="country/USA"
  childPlaceType="State"
  variable="Count_Person"
  colors="blue purple red"
></datacommons-map>

[dwnoble]

Very nice!