Allow selected (custom/standard) fields to be updated #157

As for now when jiralert creates an issue it is able to optionally assign mapped values for standard or custom fields, yet when the alert's values are updated jiralert won't update any of these fields.

We have a use case such as this:
Disk usage alert's $value is assigned to an Annotations.DiskUsage, in jiralert it is mapped to some custom field and we want jiralert to update it periodically so that Jira filters or other automations based on that custom field could be possible.

Such a feature could enrich prometheus->jira integration possibilities.