Replace WebAssembly feature CLI options with --wasm-features.

This commit hides the existing WebAssembly feature CLI options (e.g.
`--enable-simd`) and adds a `--wasm-features` flag that enables multiple
(or all) WebAssembly features.

Features can be disabled by prefixing the value with `-`, e.g.
`--wasm-features=-simd`.

src/commands/wast.rs

@@ -7,12 +7,19 @@ use structopt::{clap::AppSettings, StructOpt};
 use wasmtime::{Engine, Store};
 use wasmtime_wast::WastContext;
 
+lazy_static::lazy_static! {
+    static ref AFTER_HELP: String = {
+        crate::WASM_FEATURES.to_string()
+    };
+}
+
 /// Runs a WebAssembly test script file
 #[derive(StructOpt)]
 #[structopt(
     name = "wast",
     version = env!("CARGO_PKG_VERSION"),
     setting = AppSettings::ColoredHelp,
+    after_help = AFTER_HELP.as_str(),
 )]
 pub struct WastCommand {
     #[structopt(flatten)]