Default to a usable random placeholder

By default, generate usable random placeholders instead of nil. Thus, differing placeholders (canvas ID's) are generated automatically. This eases having multiple charts on a page.

So, it enables easier coding like this:
```ruby
chart1 = LazyHighCharts::HighChart.new
chart2 = LazyHighCharts::HighChart.new
# Code to set chart options here.
high_chart(chart1.canvas, chart1)
high_chart(chart2.canvas, chart2)
```