Add a few common configurations so people don't have to set up a environment and build the whole font from scratch for some simple variations.

Author: barrycohen

prebuilt/hack-droid/HackDroid-Bold.ttf

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+Designed to mimic the feel of Droid Sans Mono with the awesome extras of Hack.
+
+Modifications applied: `ampersand-nogap` `g-loop` `i-slab` `l-slab` `one-noslab` `parentheses-rounder` `percent-traditional` `zero-forwardslash`
+
+Font renamed to `Hack Droid`.
+
+Hack (original):
+
+![hack-sample](https://user-images.githubusercontent.com/23739434/75803136-d38cd380-5d75-11ea-9b9b-4a09642447e9.png)
+
+Droid Sans Mono:
+
+![droid-sans-mono-sample](https://user-images.githubusercontent.com/23739434/75803134-d2f43d00-5d75-11ea-96d5-728bf249d930.png)
+
+Hack Droid:
+
+![hackdroid-sample](https://user-images.githubusercontent.com/23739434/75803135-d38cd380-5d75-11ea-8edf-5fb1cc1b5eda.png)

prebuilt/hack-droid/HackDroid-BoldItalic.ttf

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+Modifications applied: `i-slab` `l-slab` `one-noslab` `zero-dotted`
+
+Hack (original):
+
+![hack-sample](https://user-images.githubusercontent.com/23739434/75803136-d38cd380-5d75-11ea-9b9b-4a09642447e9.png)
+
+Variation:
+
+![zero-dotted-slabs-sample](https://user-images.githubusercontent.com/23739434/75803139-d4256a00-5d75-11ea-9920-695ae71a02be.png)

prebuilt/hack-droid/HackDroid-Italic.ttf

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+Modifications applied: `zero-dotted`
+
+Hack (original):
+
+![hack-sample](https://user-images.githubusercontent.com/23739434/75803136-d38cd380-5d75-11ea-9b9b-4a09642447e9.png)
+
+Variation:
+
+![zero-dotted-sample](https://user-images.githubusercontent.com/23739434/75803137-d4256a00-5d75-11ea-90b5-222e2825600e.png)

prebuilt/hack-droid/HackDroid-Regular.ttf

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+Modifications applied: `i-slab` `l-slab` `one-noslab` `zero-forwardslash`
+
+Hack (original):
+
+![hack-sample](https://user-images.githubusercontent.com/23739434/75803136-d38cd380-5d75-11ea-9b9b-4a09642447e9.png)
+
+Variation:
+
+![zero-forwardslash-slabs-sample](https://user-images.githubusercontent.com/23739434/75803142-d4be0080-5d75-11ea-9648-40a6a5224c75.png)

prebuilt/hack-droid/README.md

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+Modifications applied: `zero-forwardslash`
+
+Hack (original):
+
+![hack-sample](https://user-images.githubusercontent.com/23739434/75803136-d38cd380-5d75-11ea-9b9b-4a09642447e9.png)
+
+Variation:
+
+![zero-forwardslash-sample](https://user-images.githubusercontent.com/23739434/75803140-d4256a00-5d75-11ea-95a9-965c4bee6f4e.png)