Add basic plugin infrastructure

Provide a way to write tarantool plugins.
The server loads all plugins at start.

Plugin declaration structure looks like:

typedef void(*plugin_init_cb)(struct lua_State *L);
typedef void(*plugin_stat_cb)(struct tbuf *out);

struct tarantool_plugin {
        int api_version;
        int version;
        const char *name;
        plugin_init_cb init;
        plugin_stat_cb stat;
        struct rlist list;
};

#define DECLARE_PLUGIN(name, version, init, stat) \
        extern "C" { \
                struct tarantool_plugin plugin_meta = { \
                        PLUGIN_API_VERSION, \
                        version, \
                        name, \
                        init, \
                        stat, \
                        { NULL, NULL } \
                }; \
        }

DECLARE_PLUGIN(PLUGIN_NAME, PLUGIN_VERSION, init, NULL);