Add ability to specify decimal separator

[LHMarshall]

It'd be great to have the ability to specify the decimal separator in the activity shiny, before exporting the bootstrap activity values to file. I think this means adding a drop-down so the user can specify "." (the default) or "," and then passing their choice along to the write.table call using the dec= option.

The reason to request this is that Distance for Windows expects the decimal separator to be the correct one for the locale the user is in, so it's expecting a , in some locales.

We need to have a user-specified option in the shiny rather than automatically sensing it because the shiny server doesn't know what locale the user is running shiny from.