Add initial version of curl plugin

plugins/curl.lua

@@ -0,0 +1,21 @@
+function tempdir()
+    local stream = assert(io.popen('mktemp --directory'))
+    local output = stream:read('*all')
+    stream:close()
+    return string.gsub(output, "\n", "")
+end
+
+function preprocess(path_or_url)
+    filename_from_url = string.match(path_or_url, '^https?://.*/(.*)$')
+    if filename_from_url then
+        local temp_directory = tempdir()
+        local new_path = temp_directory .. "/" .. filename_from_url
+
+        -- TODO: how to prevent shell injection bugs?
+        os.execute("curl --silent '" .. path_or_url .. "' --output '" .. new_path .. "'")
+
+        return new_path
+    else
+        return path_or_url
+    end
+end