Implement reading gzipped WARC files

[jhwgh1968]

Implements the read half of #8.

[jedireza]

Thanks! Code looks good. I test drove this and it worked as expected. I did see warnings about the unused imports. Let's drop those and we can land this.

warning: unused imports: `CONTENT_LENGTH`, `WARC_DATE`, `WARC_RECORD_ID`, `WARC_TYPE`
 --> src/warc_types.rs:3:21
  |
3 | use crate::header::{CONTENT_LENGTH, WARC_DATE, WARC_RECORD_ID, WARC_TYPE};
  |                     ^^^^^^^^^^^^^^  ^^^^^^^^^  ^^^^^^^^^^^^^^  ^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

[jhwgh1968]

Fixed, @jedireza!

[jedireza]

Thanks again. Published as v0.2.1.