Enable template options

# What it does
Enables template options via --opt VAR=SOMETHING

# What it does NOT
Break how everything used to work, so old templates still work

# Description
```
$ okconfig addtemplate www.okconfig.org --template http \
        --opt VIRTUAL_HOST=vhost.okconfig.org \
        --opt SEARCH_STRING=something
Saved /etc/nagios/okconfig/hosts/default/www.okconfig.org-http-vhost.okconfig.org.cfg
```

Contents of the file (/etc/nagios/okconfig/hosts/default/www.okconfig.org-http-vhost.okconfig.org.cfg) become
```
define service {
	use			okc-check_http
	host_name		www.okconfig.org
	contact_groups		default
	service_description	HTTP vhost.okconfig.org
	#check_command		okc-check_http

	__VIRTUAL_HOST		vhost.okconfig.org
	#__URI			/
	__SEARCH_STRING	something
	#__RESPONSE_WARNING	2.0
	#__RESPONSE_CRITICAL	10.0
	#__PORT			80
	#__ON_REDIRECT           follow

}
```

Then there is the configuration for the template options.
/usr/share/okconfig/examples/http.yml
```
filename:   "{HOSTNAME}-http-{VIRTUAL_HOST}.cfg"
parms:
    VIRTUAL_HOST: 
        default:    k:HOSTNAME
        summary:    Virtual host to query for
    URI:
        default:    /
    SEARCH_STRING:
        default:    
    RESPONSE_WARNING:
        default:    2.0
        summary:    Response time in seconds till warning
    RESPONSE_CRITICAL:
        default:    10.0
        summary:    Response time in seconds till critical
    PORT:
        default:    80
    ON_REDIRECT:
        default:    follow
        summary:    Action to on redirects, eg <ok|warning|critcal|follow|sticky|stickyport>

```

It still uses the example file which now looks like this:
```
define service {
	use			okc-check_http
	host_name		HOSTNAME
	contact_groups		GROUP
	service_description	HTTP ^VIRTUAL_HOST
	#check_command		okc-check_http

	__VIRTUAL_HOST		^VIRTUAL_HOST
	#__URI			^URI
	#__SEARCH_STRING	^SEARCH_STRING
	#__RESPONSE_WARNING	^RESPONSE_WARNING
	#__RESPONSE_CRITICAL	^RESPONSE_CRITICAL
	#__PORT			^PORT
	#__ON_REDIRECT           follow

}
```


I am not convinced that ^ is a good way to indicate template options. Maybe better, and less error prone to use {VARIABLE}
and then use built-in python's string.format():
```
template_output = template_output.format(**template_opts)
```