Add $article_width variable for renderText option

[DhruvaSambrani]

Add runtime var to send the actual width of the article to renderText command by text replacement

This allows renderText to have $article_width anywhere in the command which will be replaced by the width of the article display window.

Code Changes-

• Add article_width variable to TextConverter
• Add function helper.h#replaceString
• Change TextConverter to replace instances of $article_width with val

Other Changes-

• Change doc/neix.1 to reflect changes in options

Fixes #21

[qw3rtty]

@DhruvaSambrani Thanks for your work!

But there are some build errors when the Unit-Tests are enabled. Could you check that and fix?

[DhruvaSambrani]

I have fixed the error by changing param order and adding a default value. Should I also add a test case for this?

[DhruvaSambrani]

@tomschwarz please check

[qw3rtty]

@DhruvaSambrani Thanks for your fix! I really appreciate your work!