options and apis to cammelcase

[harttle]

One of the main purpose of this repo is provide a shopify-compliant template engine in JavaScript. Now I find something wrong with the code base: there're more and more shopify-like or ruby-like code and they look weird. So here's the plan:

1. Rename snake_cased liquidjs options (such as strict_filters) to camelCase (like strictFilters), which is widely used in JavaScript world and so in this repo.
2. Rename API/Interfaces into camelCase, such as liquid_method_missing should be liquidMethodMissing
3. JavaScript toString is used rather than liquid-like inspect for object output. Differences between toString and to_s are documented here
4. Behavior for math filters will stick to JavaScript Number, i.e. {{0.3 | minus: 0.1}} will output "0.19999999"

That does not mean liquidjs is no longer compatible to shopify/liquid, but it will do more to embrace the JavaScript way. Despite the APIs and options will be different, the following behaviors/rules will always remain the same:

1. the template syntax will be fully compatible
2. the tags behaviors will be fully compatible
3. the filters/outputs will be almost compatible, I'll do my best to resolve or claim the language differences like undefined, null, NaN.

From now on snake_case will be deprecated, and will be remove in the next major version, which is 8.0.0