Add a mechanism for adding host-specific files

[twpayne]

Currently chezmoi will populate all files. It is likely that some files will only be required on certain machines. For example, you may need ~/company.conf on your work machine, but this file should not exist on your home machine. At the moment, you can make ~/company.conf empty on your home machine using a template:

{{- if .chezmoi.hostname eq "work-laptop" }}
# contents of company.conf
{{- end -}}

but this will still result in an empty file on other machines. We need a mechanism to only populate files that should be present.

For config files. an empty file is more-or-less equivalent to an absent file. Perhaps we can use this.