Replicate datasheets to LAN peers #8

If 2 gshmm instances are in a LAN, they should be able to see each other. An initial implementation could simply show somewhere in the interface that another peer has been found.

Hitting d for [d]ownload would then download all the peers datasheets. Once the machinery of finding / downloading is there, we can look into more nuanced exchange of relevant PDFs and such.

This means we also might need a [r]: rescan for walking the directory for new pdfs that have been downloaded.

Something like https://github.com/schollz/peerdiscovery is probably the easiest and seems reliable. We could automatically generate a name for each peer. This goroutine could always be running and feeding new peers to the model which can be passed to the UI thereafter.

I'm not sure what the UI would like for multiple peers and choosing what to download but I won't worry about that for now.