[TW-1821] Escaping of '/'s in modify/substitution command confuses the parser

_Johannes Schlatow on 2016-06-23T12:11:18Z says:_

This is related to TW-295. Although the test case is working properly, I noticed that the parses does not recognise the substitution in other (corner) cases.

The test case works by substituting '/two/' with "TWO" by using the following command:
```
task 1 modify /\\/two\\//TWO/
```

However, if I only want to match for '/two', the parser does not recognise the /match/replace/ pattern, i.e.:
```
task 1 modify /\\/two/TWO/
```
replaces the entire description with '//two/TWO/'.