Adding -H/--headers option for custom headers

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Flag to pass custom header(s) to server, just like curl
Usage examples

• -H "header1:value1
• -H "header1:value1" -H "header2:value2" -H "header3:value3"
• -H "header1:value1, header2:value2, header3:value3"
• -H headers.txt

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Commit 7c5e707 brings -b/--cookies <data> option 👍
For this addition, we need to implement the following feature

• either specify cookies or a file with the -b/--cookies <filename> flag

Moreover, we still need to implement curl-like options

• -c/--cookie-jar <filename>
• -H/--header <header|filename>