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/run.rs

@@ -68,9 +68,15 @@ fn parse_preloads(s: &str) -> Result<(String, PathBuf)> {
     Ok((parts[0].into(), parts[1].into()))
 }
 
+lazy_static::lazy_static! {
+    static ref AFTER_HELP: String = {
+        crate::WASM_FEATURES.to_string()
+    };
+}
+
 /// Runs a WebAssembly module
 #[derive(StructOpt)]
-#[structopt(name = "run", setting = AppSettings::TrailingVarArg)]
+#[structopt(name = "run", setting = AppSettings::TrailingVarArg, after_help = AFTER_HELP.as_str())]
 pub struct RunCommand {
     #[structopt(flatten)]
     common: CommonOptions,