Add buffer x

Hi!  

Newish githubber here, so hopefully I'm doing this right.  

I love this concept and when using it I was noticing the occasional text-cut off that others have seen.  I added a "bufferX" option to allow for some minor tweaking to the svg viewport width if needed to get the headlines to squeeze in.  Maybe "bufferX" is not the best name for it, but this could be helpful for allowing some flexibility with different web fonts.