parsers: lines: support multiple occurrence of blocks to parse

[rmilecki]

So far lines parser was looking for only 1 block defined by "start" and
"end" RegEx-es. Some invoices may have lines of the same set in muliple
blocks. They can be separated by some random content or page footer &
header.

To support such cases use "start" and "end" to find as many blocks to
parse as possible.

This is (hopefully) cleanly implemented by:
1. Renaming parse() to parse_block() and making it work with a single
    block (already extracted from invoice content)
2. Making new parse() find blocks one by one

This feature has been requested as a way of dealing with some multi-page
invoices.

[rmilecki]

This feature was requested by @noxqs in the #372

I added new test to:

1. Make this use case easier to understand
2. Make sure it gets tested, it works and doesn't get broken later

[bosd]

Thanks for this contribution,
Since, there is already some work ongoing on the lines parser for multiline support in #378 and #417.
I will refrain from reviewing this, for now. until that functionality has been merged.
(will check then, if these code changes are compatible with each other)

One suggestion though, It would be nice to have some kind of feedback in the logger when when there is no match on the line start or line end regex. (This is also unavailable before this PR)

[bosd]

@rmilecki Can you fix the conflicts?

[rmilecki]

@bosd: rebased & added warnings for parsing problems

[rmilecki]

@bosd: I see you rebased this again, my first thought was you want to merge or approve this. Can I ask if you had a chance to review those changes?

[bosd]

@rmilecki I'm about to give my approval on this one.
My desire is to have the ability to parse multiple different blocks by the line parser.
propably better to have that in a separate pr, to keep small incemental updates. As this one is about parsing multiple similar blocks. Will make a separate pr, with the real invoice sample files, to show you the challenge I'm trying to attack.

[bosd]

LGTM

[rmilecki]

Will make a separate pr, with the real invoice sample files, to show you the challenge I'm trying to attack.

Please do! Waiting for it!