Simplify API

[kamilturek]

The package can now be used like below:

zpl.ToPNG([]byte("^xa^cfa,50^fo100,100^fdHello World^fs^xz"))
zpl.ToPDF([]byte("^xa^cfa,50^fo100,100^fdHello World^fs^xz"))

Which are just convenience wrappers around:

zpl.ConvertBytes(
    []byte("^xa^cfa,50^fo100,100^fdHello World^fs^xz"),
    WithOutputFormat(zpl.PNG)
)
zpl.ConvertBytes(
    []byte("^xa^cfa,50^fo100,100^fdHello World^fs^xz"),
    WithOutputFormat(zpl.PDF)
)

Which are just convenience wrappers around zpl.Convert that operates on buffers.