Feature: Type -> Component mapping with weakdeps

[sjkelly]

When #195 is merged it should be possible to write an extension such that some common Types have trivial component methods:

df = DataFrame(data) 

app = dash()

app.layout = dash_datatable(
    data = map(eachrow(df)) do r
      Dict(names(r) .=> values(r))
    end,
    columns=[Dict("name" =>c, "id" => c) for c in names(df)]
)

Could become:

df = DataFrame(data) 

app = dash()

app.layout = dash_datatable(df)