[sbryngelson]

Closes #15.

Adds an opt-in leading-ampersand option (default true, so existing output is unchanged). With leading-ampersand = false the optional leading & is dropped from continuation lines and the alignment is kept:

call s_foo(x, &
           y, z)

It is a final pass, so every earlier pass still sees the canonical & form. The & is kept where Fortran requires it (a continued character literal) and where dropping it would expose a #, !, $: or @: line to the parser.

Checked on 335 real Fortran files: the only change between true and false is the removed & (14,807 lines), line counts are identical, and gfortran reports the same diagnostics for both outputs. Tests are in tests/config_integration.rs plus a unit-test module in src/formatter.rs (the string-literal guard is only reachable there, because ffmt rejoins split strings first). Documented in docs/configuration.md.

cargo fmt --check, cargo clippy --all-targets -- -D warnings and cargo test pass locally.

Note: this touches src/formatter.rs near #16 and will conflict textually with it (adjacent helper functions). It is based on master, not stacked; I will rebase it after #16 merges.