Add command-line option to fetch only new messages from first n pages

[moba]

Currently, glsfetch explores all inbox pages to gather a list of new messages every time it runs. When used in a regular fashion, eg. a cronjob, it is likely unnecessary to crawl all pages in the inbox. Instead, maybe the default behavior should be to only fetch new messages from the first page, with command line arguments if one wants to grab documents from n>1 pages.