added feature to set global options

My attempt at incorporating HighChart global options. Resolves #149. 

Option can be set in the controller with:

```ruby
@chart_globals = LazyHighCharts::HighChartGlobals.new do |f|
  f.global(useUTC: false)
  f.lang(thousandsSep: ",")
  f.colors(["#90ed7d", "#f7a35c", "#8085e9", "#f15c80", "#e4d354"])
end
```

Then printed in the view:
```ruby
<%= high_chart_globals @chart_globals %>
```