Execute a command to initialise some stuff

[siketyan]

ghr clone siketyan/test --init npm

will run npm i in the repository directory after cloning.
It requires configuration before using the feature.

[initializers.npm]
cd = { cmd = "npm", args = ["i"] }
default = { cmd = "npm", args = ["ci"] }

With the configuration above, ghr will execute npm i after ghr cd, otherwise npm ci.