Visualisation editing

[mare5x]

Closes #107

Usage:

plotA1 = thread.plot()
plotA2 = plotA1.edit("use a line chart")

# Forking
plotA3_1 = plotA2.edit("add tooltips")
plotA3_2 = plotA2.edit("make it orange")

# New plot independent of plotA
plotB1 = thread.plot("visualize it")
plotB2 = plotB1.edit("add a legend")

Forking (and chaining) works naturally since every plot and edit object is immutable and stateless (unlike Pipe.ask).