WIP: making error locations in julia-repl clickable

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I scratched a long-standing itch and made locations in error messages "clickable" in julia-repl. Not yet merged into master, the change is in the clickable-locations branch.

Testing is needed because of some hacks (again, I am not an Emacs expert), I will see if there are issues then merge it. This is what it looks like, those red and orange lines take you to the source:

julia> include("/tmp/Foo.jl")
ERROR: LoadError: UndefVarError: T not defined
Stacktrace:
 [1] include_from_node1(::String) at ./loading.jl:576
 [2] include(::String) at ./sysimg.jl:14
while loading /tmp/Foo.jl, in expression starting on line 9

julia>