Support character escaping

Currently there is no way to print a literal format character, because [qdate:to_string_worker/4](https://github.com/choptastic/qdate/blob/master/src/qdate.erl#L126) aggressively parses them before [ec_date:format/3](https://github.com/erlware/erlware_commons/blob/master/src/ec_date.erl#L408) can escape them.

```erlang-repl
1> qdate:to_string("Y").
"2014"
2> qdate:to_string("\\Y").
"\\2014"
```

This patch will do a same thing as `ec_date`, escaping format character with `\\`.

```erlang-repl
1> qdate:to_string("Y").
"2014"
2> qdate:to_string("\\Y").
"Y"
3> qdate:to_string("Z").
"+0"
4> qdate:to_string("\\Z").
"Z"
```