Introduce MUTT_TOKEN_BACKTICK_VARS to expand variables within backticks

Introduce MUTT_TOKEN_BACKTICK_VARS to expand variables within backticks

This is a new flag to mutt_extract_token which triggers variable
expansion within backticks.

With this flag:

```
:set my_foo="`echo foo $pager bar`"
:set ?my_foo
foo builtin bar
```

Without this flag:

```
:set my_foo="`echo foo $pager bar`"
:set ?my_foo
foo  bar
```

This flag is currently only passed when parsing a `set` command.