Fix: cross-chain Meta Evidence

[hbarcelos]

The recent updates in ERC-1497 introduced new parameters to make dynamic scripts and evidence display interfaces less fragile, removing the need to hard-code JSON-RPC endpoints.

One particular edge case relates to cross-chain Meta Evidence, when the JSON file should contain a hard-coded arbitrableChainID to indicate which chain it belongs to. However this created a problem when calling Archon#arbitrable.getMetaEvidence() because the caller cannot know the value of arbitrableChainID beforehand in order to be able to inject the proper arbitrableJsonRpcUrl into the dynamic script.

To overcome that issue we are introducing a new option for Archon#arbitrable.getMetaEvidence() named getJsonRpcUrl, which is a callback in the form:

options.getJsonRpcUrl(chainId: number) => string

This new option will allow archon to obtain a JSON-RPC URL from arbitrableChainID after it downloads the original file so it can be injected into scriptParameters automatically.

This change is supposed to be non-breaking and should not change the current behavior when arbitrableChainID is not defined.

[0xferit]

Looks good but couldn't we use shorthand notations? It looks like too much code.

What I mean is: A = A || B

[hbarcelos]

Done.