Export funcMap

Suppose I pre-compile amber templates with [amberc](https://github.com/eknkc/amber/tree/master/amberc): `amberc input.amber >template.tmpl`

Then using just `html/template`:

```go
tmpl := template.Must(template.ParseFiles("template.tmpl"))
```

I have to also `tmpl.Funcs(amber.funcMap)` which I can't until funcMap is lower-cased