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

[DhruvaSambrani]

Sorry for this, but closing this PR to open another PR from another branch