Modify history state instead of overwriting it

[ema-fox]

Please consider only modifying the history state and not completely overwriting it.

I haven't tested it but[1] changing this line:

clerk/src/clerk/core.cljs

Line 125 in a78d3bd

state {:scroll-top scroll-top}]

to:
state (assoc (js->clj (get-history-state)) :scroll-top scroll-top)]
should make Clerk play nice with other code that also makes use of the history state.

[1] I have tested it now and it seems to work.