feature: literal subsitution

# Literal Substitution

When we're declaring a JSON configured file, it allows string literals with embedded expressions in them (using the ES6 delimiter). This is often referred to as string interpolation - substitute template string for environment variables, e.g.:
```
{
  "apps": [
    {
      "name": "Substitution-${ MODE | substr(5, 3) | capitalize }",
      "script": "literal-${ MODE | lower }.js",
      "args": [
        "${ MODE | upper }"
      ],
      "log_file": "${ MODE | lower }.log",
      "error_file": "${ MODE | lower }-err.log",
      "out_file": "${ MODE | lower }-out.log",
      "pid_file": "${ MODE | lower | replace('-\\\\w+$', '') }.pid",
      "merge_logs": true,
      "log_date_format":"${ LOG_PREFIX | def('YYYY-MM-DD HH:mm') }",
      "env_dev": {
        "MODE": "Subs-dev"
      },
      "env_pro": {
        "MODE": "Subs-pro",
        "LOG_PREFIX":"YYYY-MM-DD HH:mm:ss Z"
      }
    }
  ]
}
```

Running command below:
```bash
$ pm2 start subs.json --env dev
```

It will be substituted to:
```
{
  "apps": [
    {
      "name": "Substitution-Dev",
      "script": "literal-subs-dev.js",
      "args": [
        "SUBS-DEV"
      ],
      "log_file": "subs-dev.log",
      "error_file": "subs-dev-err.log",
      "out_file": "subs-dev-out.log",
      "pid_file": "subs.pid",
      "merge_logs": true,
      "log_date_format":"YYYY-MM-DD HH:mm",
      "env_dev": {
        "MODE": "Subs-dev"
      },
      "env_pro": {
        "MODE": "Subs-pro",
        "LOG_PREFIX":"YYYY-MM-DD HH:mm:ss Z"
      }
    }
  ]
}
```

## Substitution Usage

```javascript
"$ { [ENVIRONMENT_VARIABLE] | [FILTERS] | ... }"
```

### Filters

The env-vars can be modified by filters, and filters can be chained together, like `$ { VAR1 | uppercase | substr(5)}`, including:

- **esc**
Backslash escape specific characters, like `'`, `"` and `\`, e.g. `$ { VAR1 | esc }`.

- **upper**
Convert the variable value to uppercase letters, e.g. `$ { VAR1 | upper }`.

- **lower**
Convert the variable value to lowercase letters, e.g. `$ { VAR1 | lower }`.

- **capitalize**
Uppercase the first letter of the variable value and lowercase the rest, e.g. `$ { VAR1 | capitalize }`.

- **def**
If the variable is `null`, `undefined` or empty, the default value will be used, e.g. `$ { VAR1 | def("var1_value") }`.

- **substr**
Return the substring of variable value, uses JavaScript's built-in `String.substr()` method, e.g. `$ { VAR1 | substr(3, 5) }` or `$ { VAR1 | substr(7) }`.

- **replace**
Returns a new string with the matched search pattern replaced by the given replacement string, uses JavaScript's built-in `String.replace()` method, e.g. `$ { VAR1 | replace('abc', 'def') }`, `$ { VAR1 | replace('-\\\\w+$', '') }` or `$ { VAR1 | replace('-\\\\w+$', '', 'ig') }`

**Notes**
- All the string parameters should be wrapped by single quote - `'`.
- Available json properties:
  - name
  - script
  - args
  - node_args
  - log_file
  - out_file
  - error_file
  - pid_file
  - log_date_format
- If you start a PM2 process by LITERAL SUBSTITUTION JSON declaration, `restart/reload/stop` must be ran with `$ pm2 restart <JSON_FILE> --env <ENV>` too (**--env <ENV> is required**).