Improve documentation for compile flags

[redfave]

Thank you so much for providing this library. It will help me to improve performance in an application.

It was tedious work for me to figure out, how to enable multiline mode.

import flpc
BASE_REGEX = r"foo-regex"
MULTILINE_MODE_ENABLED = 2  # stupid magic number, see source code: https://github.com/itsmeadarsh2008/flpc/blob/542fd3a313a5875e862b3210b88fa5bea59f1b66/src/lib.rs#L93
BASE_PATTERN = flpc.compile(BASE_REGEX, MULTILINE_MODE_ENABLED)

This is not beginners friendly as it requires having a look at the Rust source code and trying to understand it. You can't expect that from an average python user.
Therefore, it would be nice to document all the available flags at least in the examples.
For the long term development, it would be better to export the flags with speaking names, like the re module does (and also to allow passing multiple of them) to ensure compatibility.

from re import compile, MULTILINE
BASE_PATTERN = compile(r"foo-regex", MULTILINE)