Shorthand notation for most used commands

For the most used Drush commands, a shorthand
notation is supported,

  c - "cache-clear all"
  d - "pm-disable ARGUMENT"
  e - "pm-enable ARGUMENT"
  v - "variable-get ARGUMENT"
  w - "watchdog-show --full --tail"
  s - "watchdog-show --full ARGUMENT"

Thus, "drush @somehost cc all" becomes "d c", when
`DHOST` is set to `somehost`.