Proposal: EEx custom delimiters

[Taro]

I'm working with EEx without HEEx. I want to expand on processing .eex. But `<% %>` is hard-coded in `EEx.Compiler` killing flexibility. The slot syntax seems to be from EJS except that EJS supports customised delimiters.

It would be great if one can change/add the delimiter without a custom engine.

Like:

```elixir

require EEx, start: ~c'{', end: ~c'}'

```

Or, at least, it could be changed on the custom engine callback level. Is such addition intended to be handled separately in `c:handle_text`?