Add a custom user-agent

[dannyhajj]

The cache warmer needs to send a proper and custom User-Agent so access logs on web servers can log the info properly and make it easier for sysadmins to know what's happening.

[sys0dm1n]

This is already implemented, correct me if I am wrong: https://curl.haxx.se/docs/manpage.html#-A
Thank you :-)

[dannyhajj]

I see. The User Agent is being sent in the curl commands. But the wget commands aren't sending the User-Agent.
Though I would recommend adding more details to it. Like the following User-Agent string:

VarnishCacheWarmer/1.0.0 (For more details visit https://github.com/sys0dm1n/varnish-cache-warmer)

[sys0dm1n]

Prompt user for custom user-agent else use default value

[dannyhajj]

I'm not sure it's a good idea to allow changing the User-Agent. Not by default at least since anyone can still edit the code and modify it.

But the idea of the user agent here is to allow a sysadmin to see what's going on when checking their access logs. And I added the link to this repo as a comment value so they can reach this repo and see what it is about.

Especially since anyone can use this to maliciously crawl a site.

[sys0dm1n]

I still think it could be a cool feature in cases where you want to run the script from different places at the same time.

We can always append the repo's URL to the custom user-agent !

What do you think?