Enable custom colors with environment variables (#248)

* Enable custom colors with environment variables

* Rename HEXYL_XYZ env vars to HEXYL_COLOR_XYZ

doc/hexyl.1.md

@@ -106,6 +106,22 @@ _FILE_
 **-V**, **\--version**
 :   Prints version information.
 
+# ENVIRONMENT VARIABLES
+
+**hexyl** colors can be configured via environment variables. The variables used are as follows:
+
+:   - **HEXYL_COLOR_ASCII_PRINTABLE**: Any non-whitespace printable ASCII character
+    - **HEXYL_COLOR_ASCII_WHITESPACE**: Whitespace such as space or newline (only visible in middle panel with byte values)
+    - **HEXYL_COLOR_ASCII_OTHER**: Any other ASCII character (< **0x80**) besides null
+    - **HEXYL_COLOR_NULL**: The null byte (**0x00**)
+    - **HEXYL_COLOR_NONASCII**: Any non-ASCII byte (> **0x7F**)
+    - **HEXYL_COLOR_OFFSET**: The lefthand file offset
+
+The colors can be any of the 8 standard terminal colors: **black**, **blue**, **cyan**, **green**, **magenta**, **red**,
+**yellow** and **white**. The "bright" variants are also supported (e.g., **bright blue**). Additionally, you can use
+the RGB hex format, **#abcdef**. For example, **HEXYL_COLOR_ASCII_PRINTABLE=blue HEXYL_COLOR_ASCII_WHITESPACE="bright green"
+HEXYL_COLOR_ASCII_OTHER="#ff7f99"**.
+
 # NOTES
 
 Source repository: