Configuration of the tag name

I am using gatsby as a static site generator, which has its own use of `graphql` to do static data queries (outside of Relay entirely).

I also have a section of this site that I would like to communicate to relay with. Sadly, `relay-compiler` makes some strong assumptions that the name of the template tag function is always `graphql`.

I would like the ability to specify a tag name aside from `graphql`, e.g. `gql` or `relayGraphql`, which would allow for both worlds to cooperate.

```sh
relay-compiler --tagName gql
```