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ARROW-6121: [Tools] Improve merge tool ergonomics

- merge_arrow_pr.py now accepts the pull-request number as a single optional argument, e.g. `./merge_arrow_pr.py 4921`.
- merge_arrow_pr.py can optionally read a configuration file located in   `~/.config/arrow/merge.conf` which contains options like jira credentials. See the `dev/merge.conf` file as example

Closes #5000 from fsaintjacques/ARROW-6121-merge-ergonomic and squashes the following commits:

5298308d7 <Wes McKinney> Handle username/password separately (in case username is set but not password)
581653735 <François Saint-Jacques> Rename merge.conf to merge.conf.sample
7c51ca8f0 <François Saint-Jacques> Add license to config file
1213946bd <François Saint-Jacques> ARROW-6121:  Improve merge tool ergonomics

Lead-authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Co-authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>