Does not cope gracefully with adding viewer data multiple times

[paulwarren-wk]

If you attempt to add viewer data to an iXBRL file that already has viewer data in it, the plugin will add a second set of viewer data. The viewer will then fail to open, with an error of "Cannot find viewer data".

The plugin should either refuse to re-add viewer data, or should replace existing viewer data.

If the viewer encounters multiple sets of viewer data, it should either pick one of them and work with that, or it should report a more descriptive error, (e.g. "Multiple sets of viewer data found")

[paulwarren-wk]

Actually, I think the "Cannot find viewer data" error seen here was a result of #282.

The script uses the first block of viewer data it finds. What I think it doesn't cope gracefully with is multiple imports of the viewer JS itself.

#295 prevents double insertion of <script> tags by the plugin, which I think is a sufficient fix.