support for custom interpolation tags

Hello,

I wanted to use swig templates on for server side rendering and reactive on the client. A problem I had was that reactive does not allow setting own interpolation tags so I needed duplicates.

Example:
```
<h1>{{ name }}</h1>
```

```
reactive.intReg(/\{{ ([^}]+)\ }}/g);
reactive.intChar('{{');
```


In this pull request I added support for setting a custom regex and character for interpolation (similar to the set, get, .. adapters). There is also a test which covers this specific use case. I guess you may want to change the public api (names).

Best,
Bo