Handle absolute paths in config

[ZipFile]

Was trying to use /tmp/air as tmp_dir but it did not work well. filepath.Join simply concatenates both components, resulting in something like /home/user/myproj/tmp/air, which is counter-intuitive.

[PhilThurston]

This is especially useful for those running air via docker because being able to set the tmp directory someplace else such as the system /tmp or even in the user's home directory would resolve issues where you have directories created that require elevated permissions to remove, manage or even ignore from git. Confusing because we can handle relative paths such as ../tmp or tmp, but not /tmp

[ZipFile]

Yep, docker with restricted write permissions was my exact use case. Anyway, rebased with the latest master since there is an interest in this PR.