Add m2e support for npm and yarn goals

[OLibutzki]

By looking into the source code I realized that the frontend-maven-plugin provides m2e support for several mojo's (Gulp, Grunt, Webpack). For all these mojos one can configure triggerfiles, srddir and outputdir.

The NpmMojo and the YarnMojo don't have these properties. Is there a specific reason why they are missing?

If not, it would be great if you add them. Beside the extended feature set it leads to a more homogeneous behaviour of the different maven goals.