MINOR: halog: Add support for extracting captures using -hdr

MINOR: halog: Add support for extracting captures using -hdr

This patch adds support for extracting captured header fields to halog. A field
can be extracted by passing the `-hdr <block>:<field>` output filter.

Both `<block>` and `<field>` are 1-indexed.

`<block>` refers to the index of the brace-delimited list of headers. If both
request and response headers are captured, then request headers are referenced
by `<block> = 1`, response headers are `2`. If only one direction is captured,
there will only be a single block `1`.

`<field>` refers to a single field within the selected block.

The output will contain one line, possibly empty, per log line processed.
Passing a non-existent `<block>` or `<field>` will result in an empty line.

Example:

    capture request  header a len 50
    capture request  header b len 50
    capture request  header c len 50
    capture response header d len 50
    capture response header e len 50
    capture response header f len 50

`-srv 1:1` will extract request  header `a`
`-srv 1:2` will extract request  header `b`
`-srv 1:3` will extract request  header `c`
`-srv 2:3` will extract response header `f`

This resolves GitHub issue #1146.