Add text-trimming functionality

[craigfowler]

It seems that all browsers except for Safari trim the text of elements, when we read text nodes. On the other hand, Safari includes leading/trailing whitespace.

I'd like these bits of functionality to trim the leading/trailing whitespace by default, unless specified otherwise (using a builder):

• CSF.Screenplay.Selenium/Queries/TextQuery.cs
• CSF.Screenplay.Selenium/Builders/QuestionQueryBuilder.cs
• CSF.Screenplay.Selenium/Builders/QuestionMultiQueryBuilder.cs
• CSF.Screenplay.Selenium/Builders/QueryPredicatePrototypeBuilder.cs

The builder will offer the ability to not-trim whitespace from the front/end of the text.

[craigfowler]

This will allow the fixing of some failing cross-browser tests (mainly Safari).