Add User-Agent to all requests by default; add arg to drop it

[craigreyenga]

Closes #378

A User-Agent header is now added to all requests:

craig@Mac> ./muffet http://localhost:6890/

GET / HTTP/1.1
User-Agent: muffet/2.11.0 (macOS; arm64)         <<----------- !
Host: localhost:6890
Accept: */*
Connection: close

Customize by using the header argument:

craig@Mac> ./muffet --header="User-Agent: Chrome" http://localhost:6890/

GET / HTTP/1.1
User-Agent: Chrome
Host: localhost:6890
Accept: */*
Connection: close

Legacy behavior (no UA):

craig@Mac> ./muffet --drop-user-agent http://localhost:6890/

GET / HTTP/1.1
Host: localhost:6890
Accept: */*
Connection: close

[craigreyenga]

This is about as fancy as the user-agent can get without importing more libraries or executing external commands.