support `IntervalSets` through weak deps

[t-bltg]

julia> using UnicodePlots, IntervalSets
julia> lineplot(identity, 0..2)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (1918166) 99.80% compared to head (30b34df) 99.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #342 +/- ## ========================================== - Coverage 99.80% 99.80% -0.01% ========================================== Files 34 35 +1 Lines 2046 2045 -1 ========================================== - Hits 2042 2041 -1 Misses 4 4 ``` | [Impacted Files](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/342?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots) | Coverage Δ | | |---|---|---| | [ext/UnitfulExt.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/342?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-ZXh0L1VuaXRmdWxFeHQuamw=) | `100.00% <ø> (ø)` | | | [ext/IntervalSetsExt.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/342?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-ZXh0L0ludGVydmFsU2V0c0V4dC5qbA==) | `100.00% <100.00%> (ø)` | | | [src/UnicodePlots.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/342?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL1VuaWNvZGVQbG90cy5qbA==) | `100.00% <100.00%> (ø)` | | | [src/common.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/342?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2NvbW1vbi5qbA==) | `99.58% <100.00%> (+<0.01%)` | :arrow_up: | | [src/interface/lineplot.jl](https://codecov.io/gh/JuliaPlots/UnicodePlots.jl/pull/342?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL2ludGVyZmFjZS9saW5lcGxvdC5qbA==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots)

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