Implement bi-directional reactions

Change the `react()` function to support bi-directional reactions.

Example of syntax:

```R
reactions <- c(
    'A + B <-> C',
    'A -> D'
)
ki <- c(
    c(5, 10),
    3
)
```

This means that there is a reaction `A + B -> C` with `k = 5`, another reaction `C -> A + B` with `k = 10`, and another one `A -> D` with `k = 3`.