`nvm unalias`: Provide a restore command when unaliasing.
`nvm unalias`: Provide a restore command when unaliasing.

Fixes #916.
lukechilds/nvm,Theodeus/nvm,creationix/nvm,creationix/nvm,slariviere/nvm,Theodeus/nvm,dfackler/nvm,edwmurph/nvm,ljharb/nvm,dfackler/nvm,PeterDaveHelloKitchen/nvm,tlevine/nvm,tlevine/nvm,slariviere/nvm,PeterDaveHelloKitchen/nvm,PeterDaveHello/nvm,PeterDaveHello/nvm,evocateur/nvm,evocateur/nvm,lukechilds/nvm,ljharb/nvm,edwmurph/nvm