0.6.0: new commands (use, do), new plugin (haskell) & other improvements

• added use command as synonym to activate/on
• added functions for plugin: plug_unpack & plug_configure
• fixed plug_install_default: it works with any file ext, not only .tar.gz
• command mk: improved logic of the creation of the environment
• added function for plugin: plug_download
• erlang-plugin: fixed sorting for ls-versions
• added to all functions from nv_common module prefix nv_
• added do command
• added option --on for mk command to active env after installation
• improved files downloading (nv_download_file):
  • used custom user-agent
  • used options for continue downloading if it was interrupted
• added option --no-meta for ls command
• improved environment's name autocompletion for rm/on/activate/use
• added function plug_check_deps for checking dependencies before plugin building
• added plugin for haskell