Add push! and append! for options. (#183)

Addesses #182, **NEEDS DOCS**.

src/options.jl

@@ -138,6 +138,17 @@ function accum_opt!(d::AbstractDict, opt::AbstractDict)
     end
 end
 
+function Base.append!(options::Options, opts)
+    for opt in opts
+        accum_opt!(options.dict, opt)
+    end
+    options
+end
+
+function Base.push!(options::Options, opts::Union{String,Pair}...)
+    append!(options, opts)
+end
+
 function dictify(args)
     options = Options()
     for arg in args

test/test_options.jl

@@ -30,3 +30,10 @@ end
     @test squashed_repr_tex(@pgf Plot({}, Table([], []))) ==
         "\\addplot[]\ntable[row sep={\\\\}]\n{\n\\\\\n}\n;" # note []
 end
+
+@testset "options push! and append!" begin
+    opt1 = "color" => "red"
+    opt2 = "dashed"
+    @test @pgf(push!({}, opt1, opt2)).dict == Dict([opt1, opt2 => nothing])
+    @test @pgf(append!({}, [opt1, opt2])).dict == Dict([opt1, opt2 => nothing])
+end