Load existing report for viewing again

[TobiTenno]

In order to support things like "Review Mode" (#22) we need to be able to load an already generated report (HTML file) back into the application.

At the most basic level, this will just include adding a button that pops a file selection dialog where the user can choose an existing HTML report to load. The loaded report can just be set in the output pane just like we do when generating a new report.

[jkuester]

One interesting consideration here is how do we validate that the selected HTML file is actually a CF report?

[TobiTenno]

add a class to the body and read in something from the head? dunno beyond that