Format change log sections with proper headings

This makes the change log sections have proper headings, which were just
normal text before.

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 ## 0.1.6 (2017-06-12)
 
-Features:
+### Features
 
 * unique IDs for all variables (user-defined variables are renamed)
 * support for hiding predicates from completed output by using `#show` statements
@@ -12,7 +12,7 @@ Features:
 * command-line option `--parentheses` to fully parenthesize the output
 * adds multiple example instances for experimenting
 
-Bug Fixes:
+### Bug Fixes
 
 * adds missing error message when attempting to read inaccessible file
 * removes unnecessary parentheses after simplification
@@ -20,52 +20,52 @@ Bug Fixes:
 
 ## 0.1.5 (2017-05-04)
 
-Bug Fixes:
+### Bug Fixes
 
 * fixes lost signs with negated 0-ary predicates
 
 ## 0.1.4 (2017-04-12)
 
-Features:
+### Features
 
 * completion of input programs (optional)
 * command-line option `--complete` to turn on completion
 
 ## 0.1.3 (2017-03-30)
 
-Features:
+### Features
 
 * support for anonymous variables
 
-Bug Fixes:
+### Bug Fixes
 
 * fixes incorrectly simplified rules with comparisons
 * fixes misleading error message concerning negated, unsupported body literals
 
 ## 0.1.2 (2017-03-23)
 
-Features:
+### Features
 
 * simplification of output formulas (optional)
 * command-line option `--simplify` to turn on simplification
 
-Bug Fixes:
+### Bug Fixes
 
 * fixes incorrectly translated choice rules with multiple elements in the head aggregate
 
-Internal:
+### Internal
 
 * explicit syntax tree representation for first-order formulas
 
 ## 0.1.1 (2017-03-06)
 
-Features:
+### Features
 
 * support for choice rules (without guards)
 
 ## 0.1.0 (2016-11-24)
 
-Features:
+### Features
 
 * initial support for translating rules in *Essential Gringo* (excluding aggregates) to first-order logic formulas
 * command-line option `--color` to autodetect, enable, or disable color output