Add epoch time macros

[marcusolsson]

This PR adds macro support for custom query params and JSON Path queries:

• $__unixEpochFrom() is substituted with the start of the time interval in Unix epoch (seconds)
• $__unixEpochTo() is substituted with the end of the time interval in Unix epoch (seconds)

Example:

The following JSON Path query:

$.items[?(@.created_at > $__unixEpochFrom() && @.created_at < $__unixEpochTo())].name

gets expanded into

$.items[?(@.created_at > 1607006005 && @.created_at < 1607017003)].name

Same thing for query parameters:

from=$__unixEpochFrom()&to=$__unixEpochTo()

becomes

from=1607006005&to=1607017003

Closes #17