Add default labels to issue templates

.github/ISSUE_TEMPLATE/report-a-bug.md

@@ -1,5 +1,6 @@
 ---
 name: Report a bug
+labels: bug
 about: Report a bug with ALE.
 
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.github/ISSUE_TEMPLATE/suggest-a-new-linter-or-fixer.md

@@ -1,5 +1,6 @@
 ---
 name: Suggest a new linter or fixer
+labels: new tool
 about: Suggest a new tool ALE can officially integrate with.
 
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.github/ISSUE_TEMPLATE/suggest-an-improvement.md

@@ -1,5 +1,6 @@
 ---
 name: Suggest an improvement
+labels: enhancement
 about: Suggest some way to improve ALE, or add a new feature.
 
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