Add NPM dependencies to plugin manifest #413

Allow addition of external NPM dependencies to the DWC plugin manifest so that the DWC plugin build script installs them before a plugin is actually built (Webpack can already deal with additional dependencies quite nicely). The current work-around is to install them manually in the DWC directory before bulding a plugin but obviously this isn't very portable. This change will be required for the new IS plugin and potentially other plugins.

Also we need to make sure those dependencies don't overlap with already installed DWC dependencies because that may create build errors due to incompatibilities (e.g. chart.js=2 vs chart.js=3).