Make it possible to set options.tmpl_path to a function returning a path...

... based on the name of a template _or_ a string containing the root path to html templates. 

The change makes it possible to override the paths or generate them on the server-side if the deployment method requires that. 

An example is the sprockets asset pipeline used by Rails which adds a digest to an asset's filename based on the contents of each individual file.

It should also take care of: https://github.com/Serhioromano/bootstrap-calendar/pull/266

A trivial example usage that is an equivalent of setting `tmpl_path` to a string (which btw. is still supported):

    var options = {
      tmpl_path: function(name) {
        return '/templates/' + name + '.html';
      },
    }

Of course, creating a function like that makes most sense if the paths are generated on the server side. 

While the pull request has no dependencies on Rails, I needed it for my Rails application. An example in Rails would be using the following code:

    var options = {
      tmpl_path: function(name) {
        var htmlAssets = <%= raw paths_to_html_templates.to_json %>;
        return htmlAssets[name];
      },
    ...
    }
    var calendar = $("#calendar").calendar(options);
    ...

Assuming the templates are under /app/assets/templates/, this is the code for the helper:

    module ApplicationHelper
      ...
  
      def paths_to_html_templates
        Hash[Pathname.glob(File.join(Rails.root, "app", "assets", "templates", "*.html")).map { |p| [p.basename(".*"), path_to_asset(p.basename)] }]
      end

    ...