Inconsistent parsing of tags

[kaklakariada]

Description

OFT parsing of tags is inconsistent as discovered by @RobertZickler in #419.

Steps to Reproduce

Create requirements in a Markdown document with the following content:

Tags: täg
Tags: -tag
Tags:
* täg
* -tag
* _

OFT rejects / ignores tags with forbidden characters when when specified in a single line, but when specifying tags in an itemized list, all characters are allowed.

Expected behavior

• OFT handles tags consistently OFT rejects tags with special characters even in an itemized list
• Documentation explicitly explains allowed characters for tags.
• OFT must reject tag _ because it conflicts with the special parameter --wanted-tags _

Background

• Tags must not contain special characters to avoid problems parsing them on the command line, when a tag starts with a dash, e.g. -tag.
• Underscore _ is a wildcard for CLI argument --wanted-tags