v0.6.4

Release 0.6.4

Bump version, document the changes since 0.6.3 in the extension
release notes, broaden the file-navigation feature line in the root
README to cover IMPORT/RESTART/GDFILE and PATHS aliases, register the
new file extensions in the supported-extensions list, and document
the new opm-flow.additionalFileExtensions setting.

The 0.6.4 release notes cover: issue #13 unquoted-string fix, bare
record-token coloring, multi-line record acceptance (MESSAGES,
VFPPROD/VFPINJ, SUMMARY list-of-name mnemonics), bare-stacked SUMMARY
mnemonics (GMWPR/GMWIN), broader keyword recognition (SUMMARY/tracer/
TVDP/performance/control-mode/FIP/UDQ), extra file-link keywords
(IMPORT/RESTART/GDFILE) with PATHS-alias expansion, 17 new file
extensions, and the additionalFileExtensions setting.