Add escaping mechanism for '#' symbol in the configuration file

This was original bug report in the #2398 by @fullstopslash, which could be summarized as failure to parse taskrc lines that contain # character (in a non-comment way), because # cannot be escaped:

```
The software currently can't handle defined filters or contexts where a tag is formed such as "#This" the command dumps everything past the # even if it's surrounded in quotes, or has a prepending backslash.

What command(s) did you run?
task context list
task

Running this bare command works.:
task +#foo

The problem only seems to happen when the tag is defined in the rc file.
```

The thread got hijacked by a discussion about the tag formats, so I am reposting this here to track the actual underlying issue :slightly_smiling_face: