Feature request: Add allow_leading_plus and skip_white_space in chars_format

[dalle]

• Remove/deprecate the defines FASTFLOAT_ALLOWS_LEADING_PLUS and FASTFLOAT_SKIP_WHITE_SPACE.
• Add allow_leading_plus and skip_white_space in chars_format.
• [Add an additional from_chars_advanced for integer types where the format options can be set.]

There may be different needs in different parts of the program using the fast_float library.

[lemire]

Pull request invited.

[dalle]

Working on a PR for this, I also found some issues with is_space and "nan"/"inf" parsing for non char strings.

[dalle]

I have created a branch (issue275-deprecate-feature-macros) which is based on changes in #279