Improve content options for each step

[jwarby]

At present, the content option of each step must be the ID of a DOM element which contains the content to display. This is misleading - this property should be the actual text, which would allow a tour to be fully defined via JSON (except for the available callbacks).

In order to maintain support for using an element's content as the step content, a optional content_selector (or similar) option should be added to each step. This would also require a decision on precedence of these two options, should the developer accidentally (or intentionally) specify both options. A warning should probably be output to the console in this case as well.

[jwarby]

Suggest a different approach here - attempt to treat the existing content parameter as a selector, and perform the following checks:

• Check if it is a valid selector. If not, use the value as the content value
• If it is a valid selector, check the length property. If 0, do as above
• Else, use the content from the matched element

This has a couple of advantages:

• Non-breaking change
• Worst-case-scenario is that the content is output as a selector