Fix tagcloud rendering behvaiour on special and wikipages

Additional content parsing is influenced by special page/template status.

An eyeball check on the following options (format=tagcloud):

1 link=all
2 link=none
3 template=TagCloud / link=none
4 template=TagCloud / link=all
5 widget=wordcloud / link=none
6 widget=wordcloud / link=all
7 widget=wordcloud / template=TagCloud
8 widget=sphere / link=none
9 widget=sphere / link=all
10 widget=sphere / template=TagCloud
bugfix (value filter does not filter out all empty values)

Added a case for empty values
Merge branch 'bugfix'
Merge pull request #16 from s7eph4n/master

bugfix (value filter does not filter out all empty values)
Update README.md
Create CONTRIBUTING.md
Update README.md
Update README.md
Update README.md
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Created new output format HighCharts
currently a single chart: FrequencyHistogram

This format is intended to be a simple way of adding specific use case charts

Added temporary solution for:
https://github.com/SemanticMediaWiki/SemanticResultFormats/issues/15