Add configurable colors for branch prefixes

Branches can now be colored based on their prefix, if it matches
a user defined prefix in the config file. If no user defined
prefix matches, then it will fallback to the defaults: green for
'feature', yellow for 'bugfix', and red for 'hotfix'. All
remaining branches will be set to the default text color.

pkg/config/user_config.go

@@ -25,6 +25,7 @@ type RefresherConfig struct {
 
 type GuiConfig struct {
 	AuthorColors             map[string]string  `yaml:"authorColors"`
+	BranchColors             map[string]string  `yaml:"branchColors"`
 	ScrollHeight             int                `yaml:"scrollHeight"`
 	ScrollPastBottom         bool               `yaml:"scrollPastBottom"`
 	MouseEvents              bool               `yaml:"mouseEvents"`