Fix output height drift

[kjin]

First of all -- thanks for the great module! It's really useful for showing colored terminal outputs in READMEs 😄

I noticed that as the number of lines increases, the height of the resulting SVG drifts (it seems like 1 pixel per line). For example, in this snippet:

const ats = require('ansi-to-svg');

console.log(ats(new Array(20).fill('.').join('\n')));

The last dot completely drops off the bottom of the SVG, resulting in only 19 dots.

I think this fix can address that. Please take a look!

[coveralls]

Pull Request Test Coverage Report for Build 21

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.872%

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Totals
Change from base Build 20:	0.0%
Covered Lines:	101
Relevant Lines:	101

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💛 - Coveralls

[F1LT3R]

Fantastic work @kjin 👊 Thank you for this!

Sorry to take so long to get to this. It's been a busy week.

I'm just checking tests here and I'll push the latest version in a minute if all looks good.