Allow a list of aliases when specifying an alias

Sometimes it is useful to run multiple commands as part of an alias, for
example when changing a setting you may wish to then run purge.

mdk run rtl

"aliases": {
  "rtl": [
    "!sed -e 's/ltr/rtl/' -i '' lang/en/langconfig.php",
    "purge"
  ],
  "ltr": [
    "!sed -e 's/rtl/ltr/' -i '' lang/en/langconfig.php",
    "purge"
  ]
}