Plugin for uim

From 0.4.6, uim supports plugin system which loads library and scheme
definition. uim's plugin consist of both scheme and library.

    ___________    (load-plugin "foo")
   |           | ------------------+--->  libuim-foo.so
   |  libuim   |                   |
   |___________|                   +--->  foo.scm

 When called (load-plugin "foo") from libuim scheme engine, libuim-foo.so
 is loaded and call 'plugin_instance_init' in libuim-foo.so first. After
 loading libuim-foo.so is succeeded, 'foo.scm' is loaded.

* For end users
 If you want to install 3rd party plugin, you have to place both the shared
 object and scheme file to ~/.uim.d/plugin/.
 For example, if you want to install "foo", you have to locate both libuim-foo.so
 and foo.scm to ~/.uim.d/plugin. And you have to add to ~/.uim as follows,

 (load-plugin "foo")

* For system admins
 If you want to install 3rd party plugin, you have to place the shared object
 to ${libdatadir}/plugin/ and place the scheme library to ${datadir}/plugin.
 For example, if you want to install "foo", you have to install libuim-foo.so
 to ${libdatadir}/plugin and foo.scm to ${datadir}/plugin. If you want to enable
 this for all users put it as follows in ${datadir}/loader.scm,

 (load-plugin "foo")

* For plugin developers
 plugin_instance_init(void): Called when plugin is being loaded. In most case,
                             initialize variables and bind scheme symbol and C
                             functions.
 plugin_instance_quit(void): Called when plugin is being unloaded.

 - Plugin's loading scheme:
  1. Plugin loading
   dlopen(libuim-foo.so) -> call plugin_instance_init -> call "foo.scm"

  2. Plugin unloading
   call plugin_instance_quit -> dlclose(libuim-foo.so)