refactor(bash/bashrc): organise `nvm` initialisation script
refactor(bash/bashrc): organise `nvm` initialisation script

Add an else branch to the `nvm` installation script (added in 3d405ce)
for an organisation purpose.

The else branch does the following things:

  1. Load nvm to the shell;
  2. Load Bash completion for nvm;
  3. Declare an alias for nvm manual update; and
  4. Declare two aliases to list installed npm packages, globally and
     locally respectively.

shoichiaizawa/dotfiles,shoichiaizawa/dotfiles,shoichiaizawa/dotfiles