Per Package configs

[Morganamilo]

This is a more complete idea of #188 and #748

The idea is to have add a packages or more accurately a bases field.

The env section already exists so with a bases field we could say:

# Applies to all packages
[env]
BUILDDIR = foo

# Only applies to spotify and google-chome
[env]
Bases = spotify google-chome
BUILDDIR = foo
PKGDEST = bar

Similarly this could also work in options:

# Applies to all packages
[options]
MakepkgConf = foo

# Only applies to spotify and google-chome
[options]
Bases = spotify google-chome
MakepkgConf = bar

Obviously only certain fields will be overrideable. It doesn't make sense to override IgnoreDevel, NewsOnUpgrade, UpgradeMenu. In fact most fields will not be overrideable which makes me think the section should be called something other than options. Maybe [override] or [base].

I think a cleaner aproch would to do something like [env.spotify] but that would mean you could only do one package at a time and it would lead to duplication as users are likely to want to set common settings for groups of packages.