This function retrieves a specific field from the package description file.
Examples
getPackageField('Title')
#> Error in getPackageField("Title"): could not find function "getPackageField"
getPackageField('Version')
#> Error in getPackageField("Version"): could not find function "getPackageField"
getPackageField()
#> Error in getPackageField(): could not find function "getPackageField"
