Bugfix #19: Erroneous "Plugin is already installed" message

Also included ISSUE_TEMPLATE.md

Author: mludowise

ISSUE_TEMPLATE.md

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+### Description
+
+[//]: # (Include a detailed description of the problem)
+
+
+### OS X Version
+
+[//]: # (e.g. 10.12.4; To find the version number, go to the Apple menu -> About This Mac.)
+
+
+### Sketch Plugin Manager Version
+
+[//]: # (To find the version number, in Sketch go to Preferences > Plugins and look for the gray number next to "Sketch Plugin Manager".)
+
+
+### Screenshot
+
+[//]: # (Include a screenshot or screen recording, if applicable)
+
+
+### Logs
+
+[//]: # (Attach a .txt file containing logs for Sketch Plugin Manager. For instructions on how to export logs:)
+[//]: # (For v1.4+: https://github.com/mludowise/Sketch-Plugin-Manager/wiki/Exporting-Logs-v1.4-or-higher)
+[//]: # (For v1.3 or lower: https://github.com/mludowise/Sketch-Plugin-Manager/wiki/Exporting-Logs---v1.3-or-lower)