Number: Implement integer & fraction digits format

Create initial number module.

Number format setup is performed by numberFormatProperties(). Number
format execution is performed by numberFormat(). This arrangement allows
subsequent setup caching for improved performance. Yet this
implementation doesn't follow ECMA-402, it's been based on
Intl.NumberFormat instance properties.

Implement integer and fraction digits format.

Allow the following integer and fraction override options:
- Minimum integer digits `minimumIntegerDigits`;
- Minimum fraction digits `minimumFractionDigits`;
- Maximum fraction digits `maximumFractionDigits`;

It accepts the following `round` options:
- "ceil";
- "floor";
- "round" (default);
- "truncate";

Ref gh-200
Closes gh-209, gh-213
README: Include number module, plus minor updates

Ref gh-200
Number: Move format integer fraction

Move format integer fraction digits into its own file and unit test it.

Ref gh-200, gh-214, gh-215
Number: Implement significant digits format

Ref gh-200
Closes gh-214, gh-215