Treat TJ input as single string

[rpoelstra]

Input to the TJ command is a single string interspersed with glyph positioning commands.
This PR adds the whole string as a single entity to the canvas.strings property.

[maxpmaxp]

The array is used intentionally to reflect the source document "mark down". ifyou need to have a single string it's better to join TJs after tha parsing.

[rpoelstra]

Hi,

Thank you for reviewing this PR.
I think you misunderstood the intention of the PR.
It does not combine multiple TJ's, but in combines the parts inside a single TJ.
Take for instance the attached PDF 29261JEGR08239 kopie.pdf.
Without this PR the (cropped) text output of the first four TJ's is:

['Pr', 'ogr', 'ess Report', 'Serial Number:', 'Softwar', 'e V', 'ersion:', 'OS V', 'ersion:',

With this PR this becomes:

['Progress Report', 'Serial Number:', 'Software Version:', 'OS Version:',

Which, I think, better reflects the markdown of the document. As the inter character spacing (kerning) is a font property and not related to the markdown.

[maxpmaxp]

Good point. I'm going to revisit it in a day or two. Thanks for the contribution!

[rpoelstra]

@maxpmaxp Can you please review these PR's? They are becoming critical and I would like to know how to proceed.