Selectively map a field and value on Jira issue based on applied GitHub labels

[mattreid]

Two use cases come to mind here, which could maybe build off #174. If it makes more sense to handle them individually, we could split this apart, but thematically they're similar to me so keeping together for discussion.

I might want to set a value on an arbitrary field in the synced Jira based on applied GH labels. Two examples:

• If an expected label is applied to a GH issue, the corresponding Jira is created with a specific Work Type value set.
• If a team filed a Continuous Improvement issue in GH and applied a known label to it, I might want its synced Jira to have the component "Continuous Improvement", but other issues in that project to not apply that component if they don't have that label.