Fix #117: Adds "switch" command

[rgcr]

Adds switch command as an alias for start <project> -a

• In options.go file basically identifies if it's switch command then enables the Attach flag and continues the original logic

if cmd.Name == CommandSwitch {
    opts.Attach = true
}

• If no session name is passed shows an error message

./smug-test switch
switch requires a project name

• If there is no a config file shows the proper error as smug start

./smug-test switch XYZ
config not found for project XYZ

• Man pages, completions and test are updated
• All tests passed with no errors

 go test .
ok      github.com/ivaaaan/smug 0.925s

[rgcr]

I forgot to mention that I took the chance to add ls as alias for smug list 😄

{
    Name:    CommandList,
    Aliases: []string{"l", "ls"},
}