add compression of content streams

[mtcxgarble]

Kind of a feature request :)

Added compression to all content streams (not very fluent in go, changes have been based on PdfDictionaryObj.build; "works for me")

Should never create an invalid PDF (/FlateDecode Filter will be applied before interpreting content). Size of compressed content stream could be bigger than uncompressed stream for very small streams, or for streams with high-entropy content. But compression will work in most cases.

Text-rich documents should profit most, as PDF Unicode-Encoding (hex-encoded UCS16) is rather verbose and should compressed comparatively well.

Maybe this should be made into an document-level option, so that compression can be turned off if needed?

Pull Request: #59

Thank you!

[mtcxgarble]

Re: #61

compressLevel can be set with .SetCompressLevel(level int) after .init() has completed (sets level to zlib.DefaultCompression).

Possible levels like in "compress/zlib", .SetCompressLevel(0) will create PDF with uncompressed streams, same as .SetNoCompression() (comfort method...)

Usage:

pdf := gopdf.GoPdf{}
pdf.Start(...) // compressLevel is now zlib.DefaultCompression
pdf.SetCompressLevel(0) // or pdf.SetNoCompression() => uncompressed

[vantaboard]

@oneplus1000 can you close this issue if it has been resolved?