Fprettify

Important notice: this release drops Python 2 support

new defaults:
- support for submodules (han190)
- formatting of `end` statements (helmutwecke)
- indentation for `forall` and `where` (helmutwecke)
- indentation rules for fypp preprocessor statements (helmutwecke)

fixes:
- support for multiline strings
- parsing of `+`/`-` in addition/subtraction

New options:
- `--config-file` for custom configuration file (ellio167)
- `--diff` (jhaiduce)
- `--exclude` to exclude files from recursive file search (jhaiduce)
- `--case` letter case formatting of Fortran intrinsics (meteokid)
- `--disable-indent-mod` disable indentation after module / program
- `--enable-decl` whitespace formatting for variable declarations (`::`)

bug fixes

- use of config files (via optional dependency `configargparse`) (contributed by dbroemmel)
- more options for whitespace formatting and line-length (contributed by dbroemmel)
- more intrinsic statements recognised (contributed by dbroemmel)
- option to replace relational statements (contributed by dbroemmel)
- alignment of `use` statements
- various bug fixes

- more customizable and more formatting options
- support for multiline strings and statement labels
- compatibility with [fypp](https://github.com/aradi/fypp.git) preprocessor directives
- various bug fixes

Mostly bug fixes