[TW-1882] Attempt to add tag instead modifies description

_John Florian on 2017-01-03T14:27:52Z says:_

I wanted to add the tag `run-iso` to a task, so I did:
`task 118 modif +run-iso`

However that changed the description as can be seen in the undo I needed:
```
The last modification was made 1/3/2017

Prior Values Current Values
description Add a CPU speed governor CLI option that employs the cpulimit package. +run-iso
entry 20160422 20160422
modified 20160422 20170103
project builder builder
status pending pending
tags feature feature
uuid 13d9b37b-53e8-4644-95cb-2bd906d353c2 13d9b37b-53e8-4644-95cb-2bd906d353c2

The undo command is not reversible. Are you sure you want to revert to the previous state? (yes/no) yes
Modified task reverted.
```

Later, I tried `task 118 modif +foo` to confirm my general syntax and that worked as expected. As a work around, I wound up using `task 118 edit`.

Also tried and failed was `task 118 modif +'run-iso'`.