HTTP fixes/refactor

[didil]

Fixes #30

• some refactorings and renamings for clarity
• autoparses the command format globalping http https://domain.com/path?query
• arguments such as --path take precedence over fields parsed from url

Some general comments:

• We could add a linter (https://github.com/golangci/golangci-lint)
• As it's customary in Go projects, we could add a Makefile with cli run, test, lint commands
• We could add a verbose mode to allow observability into the request that is posted by printing the full post data before posting to globalping API
• I find that there is too much usage of global/package level variables (for example the "opts" variable, "path", etc), it makes unit testing harder, it's also error prone as it's more difficult to follow the code logic and it's easy to shadow the variables without realising. This could benefit from a refactor