outputFileMask option should override '_page'

[Zirafnik]

outputFileMask option should overwrite the whole part of the name before '_{num}'. Currently the output equals to:

{
    outputFileMask: 'image',
}

// Output: image_page_1.png

What I believe should be more appropriate:

{
    outputFileMask: 'image',
}

// Output: image_1.png

The _page part of the name is unnecessary and because I cannot override it while creating the images, I have to rename all of them again afterwards (which is obviously less than optimal).

Additionally, perhaps a different name or a more detailed documentation for this option could be added, as the documentation was not at all clear in what happens.