Allow the default type map to be overrided from config

[adelsz]

Config should have a new typeMap field:

{
  typeMap: {
    bigint: 'number' | 'bigint' | 'string',
    interval: 'iso-string' | 'interval' | 'string'
  }
}

This field allows to select a parser and type for each postgres type passed and returned.
pgTyped will then automatically handle parsing this type at both runtime and compile-time.

[logan12358]

This mapping can be achieved in node-postgres with the pg.types.setTypeParser mechanism. If I understand correctly, the BigInt mapping might become default for node-postgres in the future: brianc/node-pg-types#78.

pgtyped implicitly relies on the type mappings of whichever IDatabaseConnection is used. Do you know if it's common to use backends other than node-postgres to provide IDatabaseConnection?

I think two big options are:

1. Support configuration over which types are emitted, but don't do runtime conversion. Add documentation for configuring backends to match.
2. Integrate more tightly with node-postgres and automatically set the client's type parser (following https://github.com/adelsz/pgtyped/milestone/1, I guess)

[adelsz]

Implemented by #476