tag feature does not support special characters

[nus-se-bot]

Describe the bug
For the tag command, the UserGuide states that tags must be a continuous word without spaces. However, when a special character is entered, an error is thrown that the tags must be alphanumeric even though it is not documented in the UserGuide.

To Reproduce
Steps to reproduce the behavior:

1. Add file
2. Use tag 1 -a t/Asas$
3. See error

Expected behavior
The tag should either be accepted or UserGuide needs to document that tag only supports alphanumeric characters

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

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Reported by: @nivenang
Severity: Low

_{[original: nus-cs2103-AY1819S2/pe-dry-run#628]}