Refactoring names from bash to shell

The refactoring is required since completion is also supported for zsh
shell.

Refactoring details:
- flag generate-bash-completion -> generate-completion
- var  EnableBashCompletion     -> EnableShellCompletion
- var  BashComplete             -> ShellComplete
- var  BashCompletionFlag       -> GenerateCompletionFlag
- type BashCompleteFunc         -> ShellCompleteFunc

help.go

@@ -182,16 +182,16 @@ func printVersion(c *Context) {
 // ShowCompletions prints the lists of commands within a given context
 func ShowCompletions(c *Context) {
 	a := c.App
-	if a != nil && a.BashComplete != nil {
-		a.BashComplete(c)
+	if a != nil && a.ShellComplete != nil {
+		a.ShellComplete(c)
 	}
 }
 
 // ShowCommandCompletions prints the custom completions for a given command
 func ShowCommandCompletions(ctx *Context, command string) {
 	c := ctx.App.Command(command)
-	if c != nil && c.BashComplete != nil {
-		c.BashComplete(ctx)
+	if c != nil && c.ShellComplete != nil {
+		c.ShellComplete(ctx)
 	}
 }
 
@@ -270,7 +270,7 @@ func checkSubcommandHelp(c *Context) bool {
 }
 
 func checkCompletions(c *Context) bool {
-	if c.Bool(BashCompletionFlag.Name) && c.App.EnableBashCompletion {
+	if c.Bool(GenerateCompletionFlag.Name) && c.App.EnableShellCompletion {
 		ShowCompletions(c)
 		return true
 	}
@@ -279,7 +279,7 @@ func checkCompletions(c *Context) bool {
 }
 
 func checkCommandCompletions(c *Context, name string) bool {
-	if c.Bool(BashCompletionFlag.Name) && c.App.EnableBashCompletion {
+	if c.Bool(GenerateCompletionFlag.Name) && c.App.EnableShellCompletion {
 		ShowCommandCompletions(c, name)
 		return true
 	}
@@ -310,7 +310,7 @@ func bashCompletionCode(progName string) string {
      local cur opts base;
      COMPREPLY=();
      cur="${COMP_WORDS[COMP_CWORD]}";
-     opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} --generate-bash-completion );
+     opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} --generate-completion );
      COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) );
      return 0;
 };