Add point-offset calculation.

index.js
  -Redefine `interpolateLineRange()` to optionally offset points from
  their line segment by a given distance.
Add unit-test for point offsets.

test/index.js
  -Add another unit test, similar to the existing one, that provides an
  `offsetDist` argument and tests the returned points.
Precompute all offset ratios.

index.js
  -Prevent redundant offset ratio computation (it'll be the same for any
  point being rendered along a given line segment) by pre-calculating
  all of them.
Define offsetPoint(), offset start/end points.

index.js
  -Offset the start and end points, which aren't accounted for the in
  `interpolateLineRange()`'s main loop, as well.
  -Define `offsetPoint()`, to cut down on repetitive code.
Clean up index.js, fix failing unit test.

index.js
  -Clean up code (remove debugging statements, etc).

test/index.js
  -Fix offset-distance test, which wasn't modified to reflect 23b12ad.
Update README.md to mention offsets.

README.md
  -Add usage examples of offsets.

line_points_interpolation.png
  -Concatenate in an image of points interpolated along a line with an
  offset.
Merge branch 'offset_calculation'

index.js
  -Implement an optional offset distance for interpolated points.