Insert buffer-file-name as FILE if pdf-view-mode is active

[fapdash]

• checks if pdf-view-mode is active
• if it's active it inserts the file name of the currently loaded pdf as PATTERN

@jeremy-compostella I know the code could be cleaner but I wanted to first check if you are generally interested in this change?

[jeremy-compostella]

I like the idea so go ahead and make it cleaner.

Also, we support both doc-view-mode and pdf-view-mode.

[fapdash]

@jeremy-compostella Ready for review.