GitHub: create issue/PR templates. Resolves #143.

.github/ISSUE_TEMPLATE.md

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+**By placing an X inside the brackets like this ```[X]``` I confirm the following:**
+
+- [ ] I have read and understood the [contributor's guide](https://github.com/monero-project/kovri/blob/master/doc/CONTRIBUTING.md).
+- [ ] I have checked that the issue I am reporting can be replicated or that the feature I am suggesting is not present.
+- [ ] I have checked opened or recently closed [pull requests](https://github.com/monero-project/kovri/pulls) for existing solutions/implementations to my issue/suggestion.
+
+---
+
+{Enter your issue here}

.github/PULL_REQUEST_TEMPLATE.md

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+**By placing an X inside the brackets like this ```[X]``` I confirm the following:**
+
+- [ ] I have read and understood the [contributor's guide](https://github.com/monero-project/kovri/blob/master/doc/CONTRIBUTING.md).
+- [ ] I have checked that another pull request for this purpose does not exist.
+- [ ] I have considered and confirmed that this submission will be valuable to others.
+- [ ] I accept that this submission may not be used and that this pull request may be closed by the will of the maintainer.
+- [ ] I give this submission freely and claim no ownership to its content.
+
+---
+
+{Enter your pull request summary here}