Allow specifying tagpr labels

[5ouma]

1. Change the PR label name

   [tagpr]
     labelName = original-label

   A label named original-label, will be added to PRs created by tagpr.
   When major or minor changes are detected,
   the labels will remain tagpr:major and tagpr:minor respectively.

2. Use the custom labels

   [tagpr]
     useCustomLabels = true
     majorLabels = major-label1,major-label2
     minorLabels = minor-label,minor-label2

   When major or minor changes are detected,
   the labels will be major-label1 and minor-label1 respectively.
   If the majorLabels or minorLabels is not set,
   the labels will remain tagpr:major and tagpr:minor respectively.

Closes #168