Add generic hashtag support

[tokee]

Hashtags are universal to the net, so parsing all text for #LooksLikeAHashtag and adding them to the keywords-field seems like an obvious feature.

We need to ensure that the tags are extracted from text before the text is stripped of punctuation and special characters and we need to agree on what a hashtag looks like. Are these all hashtags?

• #Hashtag
• #Hash-tag
• #Hash.tag
• #Hash_tag
• #Hash/tag
• #Hashtag.
• #Hashtag2000
• #2000
• #$2000
• #☃︎
• #_
• #HashtagWhichIsExceedinglyLongSoMentallyAddAbout2000ExtraCharacters

So valid characters and maximum length, I guess?