Feature: graphType text to display text in the canvas

[mauriciopoppe]

There's a new graphType: 'text' used to display random text anywhere in the graph, the text is bound to the usual zoom/pan operations so it remains anchored to its location.

See the main page for an example.

functionPlot({
  target: '#text',
  data: [
    {
      graphType: 'text',
      location: [1, 1],
      text: 'hello world'
    },
    {
      graphType: 'text',
      location: [-1, -1],
      text: 'foo bar',
      attr: {
        'text-anchor': 'end'
      }
    }
  ]
})

Fixes #253