Retain original config file format

[karalabe]

Previously a trash --update overwrote any existing config file without retaining its original format (#46). This had the unfortunate side effect that an original yaml config file was swapped out with a flat conf type. A proposal to fix this was done in #50, but adding a new flag is a redundant operation since the code already knows what the original format was. This PR ensures that the original config file format (yaml or conf) is retained inside the Conf struct, allowing to serialize back to the same format on a dump.

Another bug was that trash recreated a new config structure from scratch on update, overwriting the old struct before the package metadata (version, external repo) could be extracted, losing all such information. A proposal was opened in #51 to fix this, but it doesn't play nice with other internal/private fields in the Conf struct. This PR offers an alternative solution by not recreating the original config struct, just its Imports field. This was all private metadata is preserved, including the importMap containing the import meta fields.

[imikushin]

LGTM