Change `preamble-eip` & `preamble-file-name` to be less annoying before an EIP number is assigned

[SamWilsn]

Requested by @MicahZoltu:

Can we special case this line:

eip: <to be assigned>

Right now it complains about being a non-negative integer. Would be better if it said nothing or mentioned editors asking for number assignment, or said "please wait for an editor to assign you a number". Actually, at the moment you get two errors on that line, one for "not a non-negative integer" and one for "this file's name should be eip-.md".

[JEAlfonsoP]

@Pandapip1 do you have any comment on this one ? Ty

[Pandapip1]

I don't see an issue with it.