Account for NaN and undefined values on Plot.Line and Plot.Area

Makes use of d3's defined() option on its Line and Area
generators to skip NaN and undefined values.

Adjusted Plot.Line and Plot.Area unit tests to create a new Plot
for each test, reducing cross-dependency between tests.
Improve tests, pass index to defined().

Had to modify d3.d.ts; will submit PR to DefinitelyTyped.
Fix linting errors.
Add NaN values to Line and Area animation quicktests.
Also handle undefined and NaN x-values on Line and Area.
Rename argument in _rejectNullsAndNaNs.

Turns out it's a projector, even though it's an IAppliedAccessor...
Merge pull request #1124 from palantir/path-generator-NaNs

Account for NaN and undefined values on Plot.Line and Plot.Area