Support a 'default' locale folder for fall-back values

[mfurtak]

Users can specify a default which applies when there is no specific value for a given language.

name ({ "en_us" => "English name", "default" => "Default name"})

will have the same effect as

name ({ "en_us" => "English name", "fr_FR" => "Default name", "it" => "Default name", "ja" => "Default name"})

similarly, a user can create a metadata folder 'default' and put in something like release_notes.txt this will be used for all languages where there isn't an explicit release_notes.txt

languages are filled in if they are either 1) used in a setting explicitly in the deliverfile 2) there is a language folder (possibly empty) in the metadata folder

[mfurtak]

@ConfusedVorlon @KrauseFx This is a clean-up of #547 to:

• rebase on to master to fix failing CI
• clean up commit history

[ConfusedVorlon]

@mfurtak you missed a line! Apologies for not spotting it when I checked this over - but it is a critical line to read defaults from the filesystem. I have sent a new pull request to fix this.