feat(fallback): by default, only fall back if you have an @ in the name

BREAKING CHANGE: auto-fallback will no longer fall back unless there was
an @ sign in the command.

README.md

@@ -83,7 +83,17 @@ $ npx -p lolcatjs -p cowsay -c \
 
 ## SHELL AUTO FALLBACK
 
-You can configure `npx` to run as your default fallback command when you type something in the command line but the command is not found. This includes installing packages that were not found in the local prefix either.
+You can configure `npx` to run as your default fallback command when you type something in the command line with an `@` but the command is not found. This includes installing packages that were not found in the local prefix either.
+
+For example:
+
+```
+$ npm@4 --version
+(stderr) npm@4 not found. Trying with npx...
+4.6.1
+$ asdfasdfasf
+zsh: command not found: asfdasdfasdf
+```
 
 Currently, `zsh`, `bash`, and `fish` are supported. You can access these completion scripts using `npx --shell-auto-fallback <shell>`.
 

auto-fallback.js

@@ -5,11 +5,15 @@ function mkPosix (opts) {
 command_not_found_${opts.isBash ? 'handle' : 'handler'}() {
   # Do not run within a pipe
   if test ! -t 1; then
-    echo "command not found: $1"
+    >&2 echo "command not found: $1"
+    return 127
+  fi
+  if ! [[ $1 =~ @ ]]; then
+    >&2 echo "command not found: $1"
     return 127
   fi
 
-  echo "Trying with npx..."
+  echo "$1 not found. Trying with npx..." >&2
   npx ${opts.install ? '' : '--no-install '}$*
   return $?
 }`
@@ -22,7 +26,11 @@ function __fish_command_not_found_on_interactive --on-event fish_prompt
   functions --erase __fish_command_not_found_setup
 
   function __fish_command_not_found_handler --on-event fish_command_not_found
-    echo "Trying with npx..."
+    if string match -q -v -r @ $argv[1]
+        echo "fish: Unknown command '$argv[1]'"
+        return 127
+    end
+    echo "$argv[1] not found. Trying with npx..." >&2
     npx ${opts.install ? '' : '--no-install '}$argv
   end