Add Multi-Selector (#18)

* multi-selector added

* removed debug printing

* Select bug fixed

* dependency to Random added

* review comments included

src/Builtins.jl

@@ -146,9 +146,10 @@ end
 
 get(textfield::TextField) = textfield.default
 
-
 """A dropdown menu (`<select>`) - the user can choose one of the `options`, an array of `String`s.
 
+See [`MultiSelect`](@ref) for a version that allows multiple selected items.
+
 `options` can also be an array of pairs `key::String => value::Any`. The `key` is returned via `@bind`; the `value` is shown.
 
 See the [Mozilla docs about `select`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select)
@@ -182,6 +183,44 @@ end
 
 get(select::Select) = ismissing(select.default) ? first(select.options).first : select.default
 
+
+"""A multi-selector (`<select multi>`) - the user can choose one or more of the `options`, an array of `Strings.
+
+See [`Select`](@ref) for a version that allows only one selected item.
+
+`options` can also be an array of pairs `key::String => value::Any`. The `key` is returned via `@bind`; the `value` is shown.
+
+See the [Mozilla docs about `select`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select)
+
+# Examples
+`@bind veg MultiSelect(["potato", "carrot"])`
+
+`@bind veg MultiSelect(["potato" => "🥔", "carrot" => "🥕"])`
+
+`@bind veg MultiSelect(["potato" => "🥔", "carrot" => "🥕"], default=["carrot"])`"""
+struct MultiSelect
+    options::Array{Pair{<:AbstractString,<:Any},1}
+    default::Union{Missing, AbstractVector{AbstractString}}
+end
+MultiSelect(options::Array{<:AbstractString,1}; default=missing) = MultiSelect([o => o for o in options], default)
+MultiSelect(options::Array{<:Pair{<:AbstractString,<:Any},1}; default=missing) = MultiSelect(options, default)
+
+function show(io::IO, ::MIME"text/html", select::MultiSelect)
+    withtag(io, Symbol("select multiple")) do
+        for o in select.options
+            print(io, """<option value="$(htmlesc(o.first))"$(!ismissing(select.default) && o.first ∈ select.default ? " selected" : "")>""")
+            if showable(MIME"text/html"(), o.second)
+                show(io, MIME"text/html"(), o.second)
+            else
+                print(io, o.second)
+            end
+            print(io, "</option>")
+        end
+    end
+end
+
+get(select::MultiSelect) = ismissing(select.default) ? Any[] : select.default
+
 """A file upload box. The chosen file will be read by the browser, and the bytes are sent back to Julia.
 
 The optional `accept` argument can be an array of `MIME`s. The user can only select files with these MIME. If only `image/*` MIMEs are allowed, then smartphone browsers will open the camera instead of a file browser.