Addons

[DariuszDepta]

Prepared module named addons, where additional components or extensions to MultiTest will be placed.
Add-ons are not required (or crucial) to use MultiTest in common testing scenarios, but provide additional functionalities, that may be used while testing CosmWasm smart contracts.

Add-ons provided in this PR:

• crate::addons::MockAddressGenerator
• crate::addons::MockApiBech32
• crate::addons::MockApiBech32m

Please consult the source code documentation to learn more about these add-ons.

Using provided add-ons, writing tests with Bech32 addresses is now simpler, e.g.:

    // prepare the application with Bech32 Api and mock address generator
    let mut app = AppBuilder::default()
        .with_api(MockApiBech32::new("juno"))
        .with_wasm(WasmKeeper::default().with_address_generator(MockAddressGenerator))
        .build(|_, _, _| {});

    // test your contract
    // ...

[DariuszDepta]

@webmaster128 This is the proposal of a concept of providing add-ons instead separate crates. Provided add-ons in this PR meet the needs reported e.g. here: #95, without copying and pasting the same code by test creators.

[chipshort]

Overall concept looks good to me. Great docs!
Just a few small things from my side.

[chipshort]

LGTM