Allow variables in commands

Syntax highlighting for these coming in next commit.

Fixes #154

tests/vars_as_commands.err

@@ -1,6 +1,6 @@
-fish: Variables may not be used as commands. In fish, please define a function or use 'eval $test'.
-exec $test
-     ^
-fish: Variables may not be used as commands. In fish, please define a function or use 'eval "$test"'.
-exec "$test"
-     ^
+fish: The expanded command was empty.
+$EMPTY_VARIABLE
+^
+fish: The expanded command was empty.
+"$EMPTY_VARIABLE"
+^

tests/vars_as_commands.in

@@ -1,8 +1,16 @@
 # Test that using variables as command names work correctly.
 
-# Both of these should generate errors about using variables as command names.
-# Verify that the expected errors are written to stderr.
-exec $test
-exec "$test"
+$EMPTY_VARIABLE
+"$EMPTY_VARIABLE"
+
+set CMD1 echo basic command as variable
+$CMD1
+
+set CMD2 echo '(' not expanded again
+$CMD2
+
+# Test implicit cd
+set CMD3 /usr/bin
+$CMD3 && echo $PWD
 
 exit 0

tests/vars_as_commands.out

@@ -0,0 +1,3 @@
+basic command as variable
+( not expanded again
+/usr/bin