impemented --runtime-args flag

[ibaryshnikov]

This pr adds the ability to pass arguments to runtime using --runtime-args flag.
At the moment I made a naive implementation of split_args function to separate arguments intended for binary from ones intended for runtime. Is it planned to use a library like clap or structopt? It may simplify the work with arguments a bit

Closes #17

[alexcrichton]

I believe the way we'll want to handle this is do some option parsing before the -- argument. Ideally this would use a full-fledged option parser, but we'll want to recognize --runtime-args as an argument to cargo wasi run, but not forward the argument to cargo run.

Put another way, --runtime-args is a flag to cargo wasi run, which means it needs to go before the --.

[ibaryshnikov]

Ideally this would use a full-fledged option parser

which one would you suggest to use here?

[alexcrichton]

I don't have one in mind per se, I haven't looked into doing this yet.

[ibaryshnikov]

I just pushed an example using structopt. What do you think about it? Is it viable to continue this way?

[alexcrichton]

So the main thing to handle here is definitely the option parsing. We'll want to be careful to not do this with simple string iteration but to instead us some form of an argument parser. For example, although weird, something could look like --pass-as-an-argument "--runtime-args" and we'd accidentally misinterpret that.

The easiest, although still difficult, solution that I can think of is to copy Cargo's help text and option parsing into cargo wasi. We'd then augment it with --runtime-args for the run/bench/test/etc cases and would use that option parser to read off the information.