Implement interpolations

This enhancement enables one to specify interpolations that are to be
applied to any values that match the search element. The replacement can
be either a simple "replace", which uses str_replace to do simple string
substitution, or an "exec", which uses the output of a command for the
replacement.

Example config:

	"interpolations": [
		{
			"search": "%branch%",
			"exec": "git symbolic-ref --short HEAD"
		},
		{
			"search": "<#LIVESERVER#>",
			"replace": "www.example.com"
		}
	],
	"servers": {
		"live": {
			"user": "operator",
			"host": "<#LIVESERVER#>",
			"webroot": "/usr/local/www/%branch%/shared/cached-copy",
			"branch": "master"
		},