config.load_langs_on_demand: permits on-the-fly loading/unloading, to facilitate proc-gen use cases

[ChanceNCounter]

This branch (squash pls) adds config.py (eventually #128) with one variable: load_langs_on_demand.

When this variable is True, the localized function caller will - if and only if a language is passed which is not loaded - load the language, call the function, and unload the language.

At the moment, this is pretty fast, though it might need to be revisited if some languages become truly bloated with resource files.

This is only desirable in certain use cases, but, for instance, the absence of this flag was an obstacle for @JarbasAl's modular text2speech package, which doesn't always know the language it wants ahead of time.

I've tested this PR against that module, and it solves the problem. I want to stress that use cases like Mycroft, which prefer to utilize at most one engine per language, will not want to set this flag.

[JarbasAl]

I approve of this PR, as discussed in chat with @ChanceNCounter sometimes we will not know what language will be needed, and in these cases the lang parameter will be passed along.

Since the user is explicitly requesting a language, he wants it loaded for sure, however as @ChanceNCounter pointed out this would bloat memory usage, eg, normalize(utt, lang="pt") for a single utterance would keep pt language data in memory, even if pt is never used again. Unloading makes perfect sense. It's up for end users to explicitly load a language if they want the performance benefits (speed) from not having to load everytime.

At same time this avoids the need for a pattern of calling load_language, func, unload_language every time we use a function

[krisgesling]

Yeah sounds like a good change, agree that the intent is there if the lang param is passed.

If Jarbas has reviewed the code I'm a +1

My only question is whether we can we add a test to cover this use case?

[ChanceNCounter]

Yeah, that's a dumb oversight! On it.