socket: too many open files

[Bhupesh-V]

Describe the bug
there is a limit to how many file descriptors can be open at a time.
See https://stackoverflow.com/questions/32325343/go-tcp-too-many-open-files-debug

To Reproduce
Steps to reproduce the behavior:

1. Run Areyouok on file with greater than 200 links (For e.g awesome-go README)
2. A lot of URLs are reported as above error

Expected behavior
All URLs should be correctly fetched

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. Windows 10]
• areyouok version: [latest]

Additional context
Use buffered channels, and add a limit for it (use ulimit -n)