support TerminalExtensions.jl

[bjarthur]

Fixes #1230 @montyvesselinov

ideally it would be nice to replace the popdisplay(GadflyDisplay) in src/terminalextensions.jl with code to put GadflyDisplay before iTerm2.InlineDisplay. as a fallback in case you have TerminalExtensions in your startup.jl, but are using a terminal which is not supported. one can get a typeof(Base.Multimedia.displays[i]), but how does one tell which module that type was defined in?

[codecov-io]

Codecov Report

Merging #1232 into master will decrease coverage by 0.55%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master    #1232      +/-   ##
==========================================
- Coverage   85.74%   85.19%   -0.56%     
==========================================
  Files          35       36       +1     
  Lines        4119     4128       +9     
==========================================
- Hits         3532     3517      -15     
- Misses        587      611      +24

Impacted Files	Coverage Δ
src/terminalextensions.jl	0% <0%> (ø)
src/mapping.jl	83.01% <100%> (-0.95%)	⬇️
src/Gadfly.jl	74.51% <33.33%> (-0.79%)	⬇️
src/theme.jl	61.4% <0%> (-8.78%)	⬇️
src/misc.jl	57.69% <0%> (-5.13%)	⬇️

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[bjarthur]

@montyvesselinov can you please see if this PR works for you? i just pushed some changes that use dispatch to put the iTerm2 display at the end of Multimedia.displays, instead of deleting the GadflyDisplay at the end, so that folks who have put TerminalExtensions.jl in their startup can still use Gadfly in another type of terminal without error.