Add config options for patch message templates

Added a new section to the global config file, template. It has two fields: message and description, pointing to files with the contents of corresponding fields for pijul record.

Analogous to git's commit.template, except that pijul separates the title from the description.

The config file is now parsed unconditionally: previously it was used only for fallbacks, but description was never set to anything, making the config the primary source.

Errors out if a file was specified in the config but could not be read.

Created by  mpevnev  on February 20, 2021
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