Math helpers

Adds `add`, `subtract`, `multiply`, and `divide` helpers:

| Template | Result |
| --- | --- |
| `{{add 1 2}}` | 3 |
| `{{add 1 2 3}}` | 6 |
| `{{subtract 2 1}}` | 1 |
| `{{subtract 3 2 1}}` | 0 |
| `{{multiply 2 3}}` | 6 |
| `{{multiply 2 3 4}}` | 24 |
| `{{divide 4 2}}` | 2 |
| `{{divide 12 4 3}}` | 1 |