Truncate long lines in source snippets

[brendanzab]

In the json-benchmark there is a 2mb canada.json almost all of of the contents being on a single line.

It might be nice to also have a Config option here for truncating leading strings from long lines, producing something like

[19389 leading characters omitted] ...
"foo", "bar", "baz",, 
                    ^ Expected `"String"`, found ","

Originally posted by @ratmice in #119 (comment)

[brendanzab]

This is similar to #125, but for the horizontal case. I think rustc does this currently - it might be useful to look at the test cases to see some examples.