Implement "format region" feature for formatters with native support

[lassik]

Continued from #17

The following formatters already have this:

• clang-format
• prettier
• swiftformat

We should add a format-all-region command to use the feature.

If you know of other formatters that also have official command line flags for formatting a region, please mention!

[drcxd]

In case you do not know about clang-format.el. I hope this could help you.

[lassik]

Thanks.

For reference, from https://clang.llvm.org/docs/ClangFormat.html:

  --cursor=<uint>            - The position of the cursor when invoking
                               clang-format from an editor integration

  --offset=<uint>            - Format a range starting at this byte offset.
                               Multiple ranges can be formatted by specifying
                               several -offset and -length pairs.
                               Can only be used with one input file.
  --length=<uint>            - Format a range of this length (in bytes).
                               Multiple ranges can be formatted by specifying
                               several -offset and -length pairs.
                               When only a single -offset is specified without
                               -length, clang-format will format up to the end
                               of the file.
                               Can only be used with one input file.

  --lines=<string>           - <start line>:<end line> - format a range of
                               lines (both 1-based).
                               Multiple ranges can be formatted by specifying
                               several -lines arguments.
                               Can't be used with -offset and -length.
                               Can only be used with one input file.

[lassik]

From prettier --help:

  --cursor-offset <int>    Print (to stderr) where a cursor at the given position would move to after
                           formatting.
                           This option cannot be used with --range-start and --range-end.
                           Defaults to -1.

  --range-start <int>      Format code starting at a given character offset.
                           The range will extend backwards to the start of the first line containing the
                           selected statement.
                           This option cannot be used with --cursor-offset.
                           Defaults to 0.
  --range-end <int>        Format code ending at a given character offset (exclusive).
                           The range will extend forwards to the end of the selected statement.
                           This option cannot be used with --cursor-offset.
                           Defaults to Infinity.

[lassik]

From swiftformat --help:

--linerange        Range of lines to process within the input file (first, last)
--fragment         Input is part of a larger file: "true" or "false" (default)

[lassik]

Done. If there are other formatters that also have region support, please send PRs updating their definitions!