Only Pkg.add("Documenter") if necessary (big speedup!)

src/plugins/documenter.jl

@@ -15,8 +15,10 @@ Generate the "docs" directory with files common to all Documenter subtypes.
 * `pkg_name::AbstractString`: Name of the package.
 """
 function gen_plugin(plugin::Documenter, template::Template, pkg_name::AbstractString)
+    if Pkg.installed("Documenter") == nothing
         info("Adding Documenter.jl")
         Pkg.add("Documenter")
+    end
     path = joinpath(template.path, pkg_name)
     docs_dir = joinpath(path, "docs", "src")
     mkpath(docs_dir)