Add ability to customize link name

[SolarLiner]

I'm trying to develop a native module for Lua 5.1 embedded in an application. The application has the Lua as a dynamic library but named simply lua as opposed to lua51, and this is causing the module to fail being required.

Not sure if at all possible, but in those cases it would be nice to have the possibility to set one from a dependent project.

[SolarLiner]

And if anybody lands on this same problem; at least on Windows, this can be worked around by using the vendored feature, disabling the module feature, and manually re-implementing the entrypoint:

#[unsafe(no_mangle)]
unsafe extern "C-unwind" fn luaopen_mymodule(state: *mut lua_State) -> c_int {
    unsafe {
        // mymodule is the function you would have applied the `mlua::lua_module` macro to
        // use `Lua::entrypoint` (without the 1) if you need the second argument passed in
        Lua::entrypoint1(state, mymodule)
    }
}