Add `-p` flag to `mkdir`, when creating temp folder

[ColdFire87]

Allow Air to create nested subdirs for the binaries.

Config:

root = "../../../../"
testdata_dir = "testdata"
tmp_dir = "temp/livereload"

[PhilThurston]

It is important to note that there are additional use cases for this request. If you set your tmp_dir = "/tmp/air" in the air.toml it will fail because it can't make a subdirectory. Additionally, even if tmp_dir = "/tmp" is set air will still create a tmp directory wherever the root is regardless even though it doesn't use it.