improve tags detection, avoid false tags

[aisbergde]

Some examples of false detected tags:

`
some content, interpreted by SilverBullet as Tags

• '#'
• Colours in default order: '#6699CC', '#004488', '#EECC66', '#994455', '#997700', '#EE99AA'.
• the words “fixes” or “closes,” for example, “fixes #14” or “closes Admonition support #67.”
• PowerRename - easy way to rename blank / space, also add example to documentation #20768
• There are C# examples abound, so we need to translate these to PowerShell.
• Code: ##869434## in den Dialer eingeben.
• An alternative when fewer colours are enough is the ##high-contrast# scheme
• which ##has more colours, but lacks a clear red or medium blue#.
• '#tags' können verwendet werden
• Chat with other Fedora users in the irc://irc.freenode.org/#fedora[Fedora IRC channel].
• [ ]

false tags.md

`

[zefhemel]

Looping @mjf here who had a very elaborate plan for parsing hash tags properly at some point.