Allow self reference without link

[aslikaya]

Fixes #84

[SamWilsn]

Why have you chosen to use by_index here instead of by_name? It might make more sense to change LinkFirst to something like:

pub struct LinkFirst<S> {
    number_header: S,
    pattern: S,
}

And get the name of the field from there instead of assuming the 0-th header is always the proposal's number.

[aslikaya]

Yes you are right, currently it looks like a requirement but the order may be subject to change in the future and this would fail. I can change and use it like by_name("eip") but do we also need to handle if it is an erc? in that case field name will be "erc" I guess

but when I try an erc on the preamble, it gave me lint errors saying :

error[markdown-refs]: references to proposals without a `category` must use a prefix of `EIP`
  --> ../test-eipw/erc-84.md
   |
23 | ERC-84
   |
error[preamble-order]: preamble has extra header(s)
 --> ../test-eipw/erc-84.md:2:1
  |
2 | erc: 84
  | ^^^ unrecognized header
  |
  = help: see https://ethereum.github.io/eipw/preamble-order/
error[preamble-req]: preamble is missing header(s): `eip`
--> ../test-eipw/erc-84.md
 |
 |
 = help: see https://ethereum.github.io/eipw/preamble-req/

It looks like they can't get into preamble fields as an erc. What to do with ercs here? They always complicate :)

I couldn't understand why update the LinkFirst struct?

[SamWilsn]

Both ERCs and EIPs use eip: as the preamble field.

Just to make things more confusing.

[aslikaya]

I changed it to by_name("eip") and shortened the code slightly.
I haven't changed the LinkFirst struct yet but I can do it too