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/bin/wasmtime.rs

@@ -6,7 +6,7 @@
 use anyhow::Result;
 use structopt::{clap::AppSettings, clap::ErrorKind, StructOpt};
 use wasmtime_cli::commands::{
-    CompileCommand, ConfigCommand, RunCommand, WasmToObjCommand, WastCommand, WASM2OBJ_AFTER_HELP,
+    CompileCommand, ConfigCommand, RunCommand, WasmToObjCommand, WastCommand,
 };
 
 /// Wasmtime WebAssembly Runtime
@@ -43,7 +43,7 @@ enum WasmtimeApp {
     /// Runs a WebAssembly module
     Run(RunCommand),
     /// Translates a WebAssembly module to native object file
-    #[structopt(name = "wasm2obj", after_help = WASM2OBJ_AFTER_HELP)]
+    #[structopt(name = "wasm2obj")]
     WasmToObj(WasmToObjCommand),
     /// Runs a WebAssembly test script file
     Wast(WastCommand),