Support quoting (sections of) left hand side in config file definitions

Currently, setting tag descriptions is done by adding an entry such as `tags.acme.description = Work done for ACME Inc.` to the `timewarrior.cfg` file (either directly or through `timew config`).

This works great for all tags with simple names, but not for tags such as `name-with-dashes` or `"name with spaces"`.

I propose allowing constructs such as:
```
tags."a non-obvious name, possibly with periods.in.the.middle".description = The description
```

At the moment, both the above and the simpler form `tags.with-dash.description = Something` fail silently (no error is reported, but timewarrior does not process any further config lines).

NOTE: My use case is with tag descriptions, but I believe this problem can be generalized for all config file definitions.