feat(Subchart): add subchart x axis tick format option

[ebencollins]

Issue

none

Details

Allows for the subchart x axis tick format to be different from the x axis tick format of the primary chart.
If the option isn't specified, it defaults to using the same format as the x axis.

Example

var chart = bb.generate({
  data: {
    x: "x",
    columns: [
	["x", "2013-01-01", "2013-01-02", "2013-01-03", "2013-01-04", "2013-01-05", "2013-01-06"],
	["data1", 30, 200, 100, 400, 150, 250],
	["data2", 130, 340, 200, 500, 250, 350]
    ],
    type: "line", // for ESM specify as: line()
  },
  axis: {
    x: {
      type: "timeseries",
      tick: {
        format: "%m-%d"
      }
    }
  },
  subchart: {
    show: true, // for ESM specify as: subchart()
    showHandle: true,
    axis: {
      x: {
        tick: {
          format: "%Y-%m-%d"
        }
      }
    }
  },
  bindto: "#timeseriesChart"
});

[netil]

@ebencollins thanks for the contribution!

[coveralls]

Coverage increased (+0.006%) to 90.765% when pulling 0daae18 on ebencollins:subx-tick-format into 8fcf61c on naver:master.