Rust API for c-lightning plugins

This is my first attempt at building a strongly typed, yet ergonomic API for the c-lightning plugin API.

Implemented features:

• Command line options
• Custom RPC methods
• Event subscriptions

Missing features:

• Hooks
• Non-blocking RPC call/event execution
• Derive macros for CmdOptions and RpcMethodParams traits and maybe a custom DeserializeOptional trait for RPC methods with optional parameters
• Human readable error reporting
• Support for stable rust (currently nightly only)

Usage

The examples show how to use the following features:

	hello_world	state	subscription
Command line options	✓
RPC methods	✓	✓
Optional RPC method arguments	✓
State		✓
Event subscriptions			✓
Logging			✓

You can test these plugins yourself by running lightningd --testnet --plugin target/debug/examples/<example_name>