Support recipe aliases

[casey]

This was requested by @ryloric in Gitter. For example:

alias b = build

build BAR:
 echo 'Building {{BAR}}!'

Given the above justfile, the build recipe could be called with build or b. Although making a recipe with no body and a single dependency (i.e. b: build) currently works, it doesn't work if the recipe has dependencies.

Allow dependencies that take parameters, a la #289, would also be a solution, but I think that a dedicated alias feature would be better.

[ryloric]

#390
The only difference from what you described is that alias b ought to be declared after build, else just will throw a compilation error.

[casey]

Fixed in #390 😎