**General**
* Moves `examples/postgres/realworld` to `examples/realworld`
* The app is now architected to support multiple DBs
* Adds feature flags for `sqlite` and `postgres` to allow user to choose
which backend to use
*NOTE* Currently it is not possible to compile with `postgres` and `sqlite`
enabled as we are using the `query!` and `query_as!` macros and they
seem to get unhappy.
* Adds CLI flags for picking the DB backend to use at runtime
* Adds schema file and implementation for SQLite for `/api/user` routes
* Adds stub routes and trait for articles and Articles entity
**Changes**
* We now use i32 instead of i64 as the user_id to get around some quirks
w/ the SQLite driver.
* Reimplements existing route handlers w/ an error handling shim so we can use
Try inside the biz logic
* *FIX* Adds a `user` key to the register user body to conform w/ realworld's
API specs
APIs were functionally tested using realworld's API test script
(https://github.com/gothinkster/realworld/tree/master/api#authentication)
* Add array of uuid, chrono, time, bigdecimal, and ipnetwork to query macro
* Comment out tests for arrays of BigDecimal
Currently arrays of BigDecimal doesn't in query macros compile.
As all of the other types work just fine, BigDecimal is simply omitted.
* Add serde_json::Value to query macros
This also adds serde_json as an optional dependency to sqlx_macros along
side a new json feature flag.
* implement for &serde_json::RawValue
* sqlx::types::Json<T> is a common type that all JSON-compatible databases can implement for,
postgres implements Json<T> as JSONB
* sqlx::postgres::types::PgJson<T> resolves to JSON
* sqlx::postgres::types::PgJsonB<T> resolves to JSONB
This introduces two new wrapper types `Json` and `Jsonb`, currently
exported by `sqlx::postgres::{Json, Jsonb}` and adds serde and
serde_json as optional dependencies, under the feature flag `json`.
Wrapping types in `Json[b]` that implement `serde::Deserialize` and
`serde::Serialize` allows them to be decoded and encoded respectivly.
