Support easier text copy from tmux terminal

Recently, [tmux is introduced as the default terminal interface](https://github.com/lablup/backend.ai/issues/96). After that, copy texts from the terminal is nearly impossible for users (paste to the terminal is okay). Drag by mouse does not copy the texts, and `Ctrl-Space` in copy mode (which can be entered by `Ctrl-B, [`) does not either seem to be working. Even if tmux's copy mode works, it would be hard for users to actually use it, aside from the fact that the copy mode copies the texts to tmux's internal buffer, not system clipboard.

I think eaiser copy should be supported for smoother user experiences. I don't know how to introduce this feature, currently, but maybe some of the following will work.
* Bind easier keys to copy by editing `.tmux.conf`
* Bind mouse behaviors to copy with drag.
* Ship with `xclip`, `pbcopy`, etc to copy texts to system clipboard.