This is a first step towards a dynamic definition of the plots we
show. For this we add a Plot javascript class, which we call to
create/update plotly plots.

This time we're preparing to tackle the stats part, or -how to
extract specific stats from the raw data and assign them to
specific plots-.

This is necessary since the application is going to need to
access (in stats.js) to the plots configuration before the plots
are created.

At this point, the only non-dynamic (as in non-externally configurable)
point is the 'datapath' field. That's for now a function in the plot
definition object where we map our raw stats object to a data point.

Eventually, this won't be necessary since this will all be defined in
Go.

This will simplify the mini websocket-based protocol we'll have,
breaking down a single do-it-all message into 2, one for the desired
plots configuration - the 'init' message- and the 'data' message,
that will just contain the data.

Upwards, as in: closer to Go, where all data processing will eventually
be performed.

For now we're still plotting runtime.MemStats though

This reverts commit 9d6be37.