make UnicodePlots print to the given io

and add support for text/plain in Plots own display_dict
fix #1514

src/backends/unicodeplots.jl

@@ -212,7 +212,7 @@ end
 
 function _show(io::IO, ::MIME"text/plain", plt::Plot{UnicodePlotsBackend})
     unicodeplots_rebuild(plt)
-    map(show, plt.o)
+    foreach(x -> show(io, x), plt.o)
     nothing
 end
 

src/output.jl

@@ -284,7 +284,10 @@ end
                 output_type = get(_best_html_output_type, backend_name(plt.backend), :svg)
             end
             out = Dict()
-            if output_type == :png
+            if output_type == :txt
+                mime = "text/plain"
+                out[mime] = sprint(show, MIME(mime), plt)
+            elseif output_type == :png
                 mime = "image/png"
                 out[mime] = base64encode(show, MIME(mime), plt)
             elseif output_type == :svg