Added option to support a custom offset on line chart axis label

[fiznool]

Fixes issue #153. The master branch chops off the axis datapoint labels when using a line chart. This pull request fixes this by adding a configurable custom offset.

See images below, notice that the 4.0% label is cut off.

Before:

After:

This is achieved with an extra offset option:

grid: {
  offset: {
    top: 10,
    bottom: 0,
    left: 10,
    right: 10
  }
}

[drichert]

👍

[tanek100]

it would be nice to have this in main code also, not in branch because i just updated the library and i lost the offset nice feature.

[atuttle]

👍

[cesutherland]

I'll get this in.