Add max_content_width option

Currently, splitting behaviour appears to be mostly dependent on max_width, unless some specific option overrides/augments this. I don't fully understand why, either.

Example, with max_width = 140 (line = 105 chars, 85 excluding indent):

                    accum = accum.checked_add(x).unwrap_or(Duration::new(u64::max_value(), 999_999_999));

Again, with max_width = 120 (longest line = 81 chars, 57 excluding indent):

                    accum = accum
                        .checked_add(x)
                        .unwrap_or(Duration::new(u64::max_value(), 999_999_999));

As I see it, there are two reasons to break long lines:

1. Available width in editors. This may or may not be an issue (it's common to have lots of spare width; some people like myself like to use variable-width fonts).
2. Readability. Too much content on one line is hard to parse.

Note that there are already a couple of more specific width rules:

1. comment_width: since comments are somewhat different to code, it makes sense to have this separate
2. inline_attribute_width: controls two things; (a) whether to use inline attributes at all (default: no), and (b) when, specifically, to allow them

Proposal:

• Add a max_content_width option; split lines when either the total line length exceeds max_width or the length minus indent exceeds max_content_width
• Replace inline_attribute_width with inline_attributes (true/false), using max_content_width to control behaviour

The issue names no files or tests. Start by locating the handling for max_width, comment_width, and inline_attribute_width in rustfmt, then trace how line splitting and inline attributes are decided. Done means implementing max_content_width as proposed, deciding the replacement behavior for inline_attribute_width, and adding coverage for both width rules.