Add additional check for sidebar width

I've added the MainWidth to the context which may be used in the future
for on the fly changing of the size of the sidebar.

README.md

@@ -23,11 +23,10 @@ Getting started
     {
         "slack_token": "yourslacktokenhere",
 
-        // add the following to use light theme, default is dark
+        // optional: add the following to use light theme, default is dark
         "theme": "light",
 
-        // Set the width of the channel list sidebar. Must be between 1 and 11
-        // as the entire width of the screen is 12 columns.
+        // optional: set the width of the sidebar (between 1 and 11), default is 1
         "sidebar_width": 3
     }
     ```

config/config.go

@@ -13,6 +13,7 @@ type Config struct {
 	SlackToken   string `json:"slack_token"`
 	Theme        string `json:"theme"`
 	SidebarWidth int    `json:"sidebar_width"`
+	MainWidth    int    `json:"-"`
 }
 
 // NewConfig loads the config file and returns a Config struct
@@ -20,6 +21,7 @@ func NewConfig(filepath string) (*Config, error) {
 	cfg := Config{
 		Theme:        "dark",
 		SidebarWidth: 1,
+		MainWidth:    11,
 	}
 
 	file, err := os.Open(filepath)
@@ -35,6 +37,12 @@ func NewConfig(filepath string) (*Config, error) {
 		return &cfg, errors.New("couldn't find 'slack_token' parameter")
 	}
 
+	if cfg.SidebarWidth < 1 || cfg.SidebarWidth > 11 {
+		return &cfg, errors.New("please specify the 'sidebar_width' between 1 and 11")
+	}
+
+	cfg.MainWidth = 12 - cfg.SidebarWidth
+
 	if cfg.Theme == "light" {
 		termui.ColorMap = map[string]termui.Attribute{
 			"fg":           termui.ColorBlack,

main.go

@@ -68,17 +68,15 @@ func main() {
 	// Create context
 	ctx := context.CreateAppContext(flgConfig)
 
-	mainWidth := 12 - ctx.Config.SidebarWidth
-
 	// Setup body
 	termui.Body.AddRows(
 		termui.NewRow(
 			termui.NewCol(ctx.Config.SidebarWidth, 0, ctx.View.Channels),
-			termui.NewCol(mainWidth, 0, ctx.View.Chat),
+			termui.NewCol(ctx.Config.MainWidth, 0, ctx.View.Chat),
 		),
 		termui.NewRow(
 			termui.NewCol(ctx.Config.SidebarWidth, 0, ctx.View.Mode),
-			termui.NewCol(mainWidth, 0, ctx.View.Input),
+			termui.NewCol(ctx.Config.MainWidth, 0, ctx.View.Input),
 		),
 	)
 	termui.Body.Align()