Increase the open file limit before writing PDFs

[tagatac]

What is changing: Check the OS-level open file limit (and adjust it if necessary) prior to writing PDF files.

Why this change is being made: There is the potential for wkhtmltopdf to need many open files (see wkhtmltopdf/wkhtmltopdf#3081). This was fixed in #14, where it depended on the order of events:

1. Stage the PDF file, getting the number of images
2. Check and adjust the open file limit
3. Flush the PDF file to disk

Number 3 was achieved via the call defer outFile.Close():

bagoup/write.go

Line 29 in 86f9b32

defer outFile.Close()

However, #29 removed Close() from the OutFile interface, collapsing Stage() and Flush() into a single function Flush() run prior to checking and adjusting the open file limit.

Related issue(s): Fixes #63

Follow-up changes needed: None AFAIK

Is the change completely covered by unit tests? If not, why not?: Yes