GitHub Pull Request Template Update (#927)

* Improve GitHub PR Template
* suggest running commands with `poetry run ...`

pyflakes here is given only certain subdirectories because `.` will try and run across all packages installed in the virtual environment, if the virtual environment is within the project's root folder (which is my common practice).

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,8 @@
 
+<!--
 # **TEMPLATE PLEASE EDIT**
-*Thank you for wanting to contribute! Please fill out this description as well as look at the checklist!*
+*Thank you for wanting to contribute! Please fill out this description as well
+as look at the checklist!*
 
 *Short block of text containing:
 - Relevant changes in text form
@@ -9,11 +11,22 @@
 - Example of usage (if applicable)
 - Example of changes to config files (if applicable)
 *
+-->
+
 ### Checklist
+
 - [ ] The code change is tested and works locally.
-- [ ] Tests pass. Your PR cannot be merged unless tests pass
+- [ ] Tests pass. Your PR cannot be merged unless tests pass. --
+  `poetry run behave`
+- [ ] The code passes linting via
+  [black](https://black.readthedocs.io/en/stable/) (consistent code styling). --
+  `poetry run black --check . --verbose --diff`
+- [ ] The code passes linting via [pyflakes](https://launchpad.net/pyflakes)
+  (logically errors and unused imports). -- `poetry run pyflakes jrnl features`
 - [ ] There is no commented out code in this PR.
 - [ ] Have you followed the guidelines in our Contributing document?
-- [ ] Have you checked to ensure there aren't other open [Pull Requests](../pulls) for the same update/change?
-- [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
+- [ ] Have you checked to ensure there aren't other open
+  [Pull Requests](../pulls) for the same update/change?
+- [ ] Have you added an explanation of what your changes do and why you'd like
+  us to include them?
 - [ ] Have you written new tests for your core changes, as applicable?