Add optional padding argument to logo tool

[rtibbles]

Summary

• Adds an optional padding argument to the logo file conversion build script
• Used for https://github.com/learningequality/kolibri-app/pull/139

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Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.226.g5a8a8842-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.226.g5a8a8842-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.226.g5a8a8842-0.4.2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.226.g5a8a8842-0.1.3-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.226.g5a8a8842.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.226.g5a8a8842-py2.py3-none-any.whl