Move rpc url from config to parameter

[konstantinzolotarev]

It makes more sense to configure RPC client using parameters (not in config)
Otherwise it's impossible to use it as real RPC client.

For example right now it's not possible to connect to 2 different EVMs at same time or reconfigure it without pain.

What do you think about something like this:

iex> config = %Ethereumex.Config{http_url: "https://localhost:8545"}
iex> Ethereumex.HttpClient.web3_client_version(config)
{:ok, "Parity//v1.7.2-beta-9f47909-20170918/x86_64-macos/rustc1.19.0"}

[hswick]

Passing around the config variable every time feels a bit clunky.

However, enabling the ability to connect to multiple EVMs does seem useful. This is solved by web3.js by simply instantiating multiple web3 instances. The only analog would be to make multiple processes, however that becomes difficult with the statically named processes under the hood.

[konstantinzolotarev]

Yep spawning new instance of rpc with custom parameters also will work.
Issue right now is that library is not usable anyhow except of connect to one JSONRPC.

[ayrat555]

@konstantinzolotarev it's possible

Ethereumex.HttpClient.web3_client_version(url: "http://localhost:8545")

#37

[konstantinzolotarev]

@ayrat555 Awesome ! Thank you !