REPL: enable visual selection

[rfourquet]

Like in ipython and emacs, when the mark is set twice in a row,
the region is set to "active", which means visually that
it is highlighted. Once it's active, setting the mark or pressing
Ctrl-G de-activates the region.

[rfourquet]

I will merge in a couple of days if no objection.

[Liso77]

Sorry for out of scope comment but maybe you could do small fix in one commit:

Home, ^A | Move to beginning of line
(from https://github.com/JuliaLang/julia/blob/master/doc/src/manual/interacting-with-julia.md )
doesn't do same thing on my linux in case of multiline edit. Home jump to first line ^A stay in current line. (same with End and ^E)

[rfourquet]

maybe you could do small fix in one commit:

Thanks for reporting, and actually you are very welcome to contribute this fix! (I won't do it in this PR, as it's unrelated).

[rfourquet]

I added basic support for "shift-selection", i.e. pressing shift with an arrow key highlights the region.
EDIT: with tmux, shift-down is apparently translated to shift-up, so this is not very usable.
EDIT2: will be fixed on tmux 2.7

[rfourquet]

There is one travis failure, which was seen elsewhere so must be not related:

Test Failed at /private/tmp/julia/share/julia/test/repl.jl:167
 Expression: wait(get_stdout) == "HI\n"
  Evaluated: "" == "HI\n"