Add size control in `pkg_search()`

[olivroy]

Fix #505

This is an attempt to fix the issue.

Let me know how to add tests.

My ad-hoc testing was

pkg_search("gist")
# 'gist' -- hits 1-4 of 4
pkg_search("git")
# 'git' -- hits 1-10 of 52
pkg_search("git", from = 2)
# 'git' -- hits 2-11 of 52
pkg_search("gist", from = 2)
# 'gist' -- hits 2-4 of 4
pkg_search("git", from = 2, size = 9)
# 'git' -- hits 2-10 of 52
 pkg_search("gist", from = 5)
x No result. :(
 pkg_search("git", from = 50)
# 'git' -- hits 50-52 of 52

[gaborcsardi]

Very cool, thanks a lot!