Feature request: return a balance

Example:

```
curl rate.sx/1Nh7uHdvY6fNwtQtM1G5EZAFPLC33B59rB
```

Returns:

```
474.27659636 BTC
```

Instead of a user specifying the coin, you could check a bunch of chains.

```
curl rate.sx/1Nh7uHdvY6fNwtQtM1G5EZAFPLC33B59rB
```

```
474.27659636 BTC + 27.00474466 BCH
```

Like this service does: https://blockchair.com/search?q=1Nh7uHdvY6fNwtQtM1G5EZAFPLC33B59rB for BTC and BCH.

Start at the rate.sx endpoint shown in the example and compare its current single-coin response with the Blockchair search behavior linked in the issue. Determine how multiple chains should be checked and represented, then verify that an address can return balances such as BTC and BCH together.

Written by the indexing model from the issue text.