Use doc.getLineHeightFactor() instead of FONT_ROW_RATIO constant in autoTableText()

[gpbmike]

When using autoTableText with an array of text strings, it would be nice for a previously set lineHeightFactor to be used.

Documentation for getLineHeightFactor: http://raw.githack.com/MrRio/jsPDF/master/docs/jsPDF.html#getLineHeightFactor
Fix would go here: https://github.com/simonbengtsson/jsPDF-AutoTable/blob/master/src/autoTableText.ts#L15

Use case:

doc.setLineHeightFactor(2);
doc.autoTableText(['First Line', 'Second Line'], 0, 0);

I would expect more space between the first and second line, but a value of 1.15 is hardcoded.

[simonbengtsson]

Looks great! Let's continue discussion in the pull request.