Custom field support

[Ruin0x11]

Adds support for custom fields.

• Main behavior is controlled via org-jira-issue-custom-fields. Can set field type, display name and location (placed under a headline or property).
• By default, the field name is the readable name provided by the Jira API, sanitized for org property use. It can be overridden by the above alist.
• The built-in org-jira issue slots like description and assignee can now be overridden by custom fields using org-jira-issue-field-overrides-alist. This is in case the organization uses a custom field for the description/etc. of the issue instead of the default. Internally the field ID is converted to and from the slot name recognized by org-jira when interacting with the Jira API.

[Nazar65]

This is a really good one, it can resolve #355

[JordanAnthonyKing]

Are there any issues with this? Looking forward to seeing it merged. Does it also support updating/progressing issues with custom fields?