Make users more reluctant to use unsafe functions

[meiersi]

One of the core goals of blaze-html is to guide the user towards writing well-formed, well-escaped Html documents. Therefore, I suggest that we improve the documentation as follows:

1. Use a separate section for the unsafe/preEscaped functions in every module documentation.
2. Mark the work 'unsafe' in bold and ensure that all reasons for why the functions is unsafe, is listed.
3. Perhaps we could even have a small intro about XSS attacks on the main documentation page. This way the users would be more sensitive to the problem.