Custom panel to monitor additional process

This is similar to #138 but maybe different enough that I'll risk opening a new issue instead of hijacking that one...

I'm playing around with modifying a TypeScript project to rely on babel instead of compilation through webpack's `ts-loader`. This splits transpiling and type checking into two process: the former through webpack and babel, the latter through `tsc` on the command line. I'd love to monitor this entirely through my webpack dashboard, ideally by splitting the large log window or by adding multiple pages, similar to the way the modules panel behaves when there are multiple bundles. I realize that this might be outside the purview of _webpack_ dashboard but I think it would be of value to plenty of developers who need to monitor multiple processes.

Thanks for your work on this project, I really enjoy it.