Validate command

[Albert221]

Create poe2arb validate command that will connect to POEditor API just like the poe2arb poe does and checks for some issues with translations:

• whether non-template languages have placeholder definitions
• whether placeholder definitions use valid types and formats
• whether there are any possible syntax errors resulting in corrupted translations
• other?

[bartekpacia]

I think it'd also be nice to find terms/strings that aren't used in the app. I got this idea from this tool I saw on our #pub-rss channel.

This is probably not in poe2arb's scope, though.