Basic i18n-tasks support with custom parser for Spree.t

[rposborne]

This is a first pass on a more long term solution, currently this requires i18n-tasks to be installed outside of the gem, as it's incompatible with spree (highline conflicts).

[rposborne]

Removing the WIP tag, as this adds support for the normalize tool which is the big leap forward that we are looking for. There are some issues regarding parsing the spree/spree repo and extracting keys that need to be present, as it requires an unreleased version of i18n-tasks that conflicts with spree/spree_cmd so can't be added to gem spec as is. @alepore @JDutil It would be great if we could get this in.

[alepore]

will take a look, we really need something like this /cc @mamhoff @tvdeyen

[tvdeyen]

Why is this inlined here?

[rposborne]

As i18n_tasks runs outside the scope of the parent application. We could add a require but it would still need to be inlined.

Basically that's what the i18n-tasks creator recommended. See glebm/i18n-tasks#170

[glebm]

i18n-tasks add-missing supports a --nil-value argument since v0.9.5, so this is no longer necessary.

[tvdeyen]

This is much greater then the self baked solution.

[glebm]

Note that a few things have been implemented in i18n-tasks that make this a bit easier:

1. add-missing now supports a --nil-value argument.
2. PatternScanner now matches the receiver, so it won't scan Spree.t calls.

[glebm]

.rb extension not necessary