add koreader plugin mechanism

This should implement #505.

frontend/apps/reader/pluginloader.lua

@@ -0,0 +1,44 @@
+local DEBUG = require("dbg")
+-- lfs
+
+local PluginLoader = {
+    plugin_path = lfs.currentdir().."/plugins"
+}
+
+function PluginLoader:loadPlugins()
+    if self.plugins then return self.plugins end
+
+    self.plugins = {}
+    for f in lfs.dir(self.plugin_path) do
+        local path = self.plugin_path.."/"..f
+        local mode = lfs.attributes(path, "mode")
+        -- valid koreader plugin directory
+        if mode == "directory" and f:find(".+%.koplugin$") then
+            local mainfile = path.."/".."main.lua"
+            local package_path = package.path
+            local package_cpath = package.cpath
+            package.path = path.."/?.lua;"..package.path
+            package.cpath = path.."/lib/?.so;"..package.cpath
+            local ok, module = pcall(require, "main")
+            if not ok then
+                DEBUG("Error when loading", mainfile, module)
+            end
+            package.path = package_path
+            package.cpath = package_cpath
+            if ok then
+                module.path = path
+                table.insert(self.plugins, module)
+            end
+        end
+    end
+
+    for _,plugin in ipairs(self.plugins) do
+        package.path = package.path..";"..plugin.path.."/?.lua"
+        package.cpath = package.cpath..";"..plugin.path.."/lib/?.so"
+    end
+
+    return self.plugins
+end
+
+return PluginLoader
+