cannot start tag names that are initial substrings of filenames in extensions dir

The syntax for `timew start` is
```
timew start [] [...]
```
but if a tag name is used that matches initial substring of a report (in `$TIMEWARRIORDB/extensions`), it will be consumed by the parser and ignored as a tag name, even though it is an argument to 'start' and should be interpreted as a tag:
```
$ echo '#!/bin/true' > extensions/foobar
$ chmod 0755 extensions/foobar

$ timew start foo
Tracking
Started 2021-10-20T22:52:40
Current 40
Total 0:00:00

$ timew start foo bar
Recorded
Started 2021-10-20T22:38:27
Ended 52:52
Total 0:14:25
Tracking bar
Started 2021-10-20T22:52:52
Current 52
Total 0:00:00
```
There happens to be no `extensions/bar*` executable so `bar` is correctly interpreted as a tag name. Using one for the first arg that isn't recognized as an extension also works:
```
$ timew start zoo bar
Recorded bar
Started 2021-10-20T22:52:52
Ended 53:02
Total 0:00:10
Note: 'zoo' is a new tag.
Tracking bar zoo
Started 2021-10-20T22:53:02
Current 02
Total 0:00:00
```
The `canonical=foobar` can be seen in debug output:
```
$ timew rc.debug=1 start foo bar
CLI Parser
_original_args
timew rc.debug=1 start foo bar
_args
word basename='timew' raw='timew' BINARY
word name='debug' raw='rc.debug=1' value='1' ORIGINAL CONFIG
word canonical='start' raw='start' ORIGINAL CMD
word canonical='foobar' raw='foo' ORIGINAL EXT
word raw='bar' ORIGINAL FILTER TAG

Tracking bar
Started 2021-10-20T23:00:32
Current 32
Total 0:00:00
```