Caching of 'get' requests moved to `i-api-request__cache`

To enable caching of  'GET' requests by default use `i-api-request`'s `cache` element:
```javascript
// content of i-api.deps.js:
({shouldDeps: [
    {block: 'i-api-request', elem: 'cache'}
]})
```
To disable caching of  'GET' request need to call method `get` with `cache: false` option:
```javascript
BEM.blocks['i-api'].get('path/to/resource', {cache: false});
```