Faulty config can be inserted, then renders everything unusable

When trying to follow the tutorial, the following commands (note the typo, I used `-` instead of `_`) rendered timew completely unusable for me, I had to manually edit the config:

```
$ python --version
Python 2.7.15rc1
$ timew --version
1.0.0
$ timew config exclusions.days.2019-05-29 off
Are you sure you want to add 'exclusions.days.2019-05-29' with a value of 'off'? (yes/no) yes
Config file /home/daniel/.timewarrior/timewarrior.cfg modified.
$ timew --help
Unrecognized construct: exclusions.days.2019-05-29 = off
```
IMHO, timew should disallow setting configuration via the commandline that makes the commandline unusable.

Start by reproducing the documented `timew config exclusions.days.2019-05-29 off` command and then `timew --help` with the resulting configuration. Trace the command-line configuration handling and make the invalid setting fail before the config is written; done means the command remains usable after rejected input.

Written by the indexing model from the issue text.