tab completion breaks on multi-word parameters

**Bug Description**
In both of these cases, tab completion stops working after the multi-word value being passed to `--input-path`. 

**Screenshots**
``` bash
qiime tools import --input-path 'foo bar' --o-
qiime tools import --input-path "foo bar" --o-
```

**Comments**
(This came up in the context of `qiime feature-table filter-samples --p-where`, but these examples are simpler.)
