Support .env config

It's a very common practice to use a `.env` file to load server configuration instead of the `config.json` file, that being said, this issue suggests to add support to loading config from `.env` file.

an example `.env` file would be as follows

```
[DB_CLIENTS]
RDBMS=postgresql
HOST=127.0.0.1
PORT=5432
DBNAME= postgres
USER=postgres
PASSWD=""
IS_FAST=true
NUMBER_OF_CONNECTIONS=1
TIMEOUT=-1.0

[REDIS_CLIENTS]
HOST=127.0.0.1
PORT=6379
PASSWD=""
DB=0
IS_FAST=true
NUMBER_OF_CONNECTIONS=1
TIMEOUT=-1.0

[LISTENERS]
ADDRESS=0.0.0.0
PORT=80
HTTPS=false

[LISTENERS]
ADDRESS=0.0.0.0
PORT=443
HTTPS=true
CERT=""
KEY=""
USE_OLD_TLS=false

[APP]
NUMBER_OF_THREADS=0
ENABLE_SESSION=true
SESSION_TIMEOUT=0
DOCUMENT_ROOT="./"
USE_IMPLICIT_PAGE=false
UPLOAD_PATH="uploads"
MAX_CONNECTIONS=100000
MAX_CONNECTIONS_PER_IP=0
LOAD_DYNAMIC_VIEWS=false
ENABLE_UNICODE_ESCAPING_IN_JSON=true
RUN_AS_DAEMON=false
HANDLE_SIG_TERM=true
RELAUNCH_ON_ERROR=true
USE_SENDFILE=true
USE_GZIP=true
USE_BROTLI=false
STATIC_FILES_CACHE_TIME=0
IDLE_CONNECTION_TIMEOUT=30
SERVER_HEADER_FIELD=drogon/version
ENABLE_SERVER_HEADER=true
ENABLE_DATE_HEADER=true
KEEPALIVE_REQUESTS=0
PIPELINING_REQUESTS=0
GZIP_STATIC=true
BR_STATIC=true
CLIENT_MAX_BODY_SIZE=1M
CLIENT_MAX_MEMORY_BODY_SIZE=64K
CLIENT_MAX_WEBSOCKET_MESSAGE_SIZE=128K
REUSE_PORT=true

[LOG]
LOG_PATH=""
LOGFILE_BASE_NAME=drogon
LOG_SIZE_LIMIT=10000000 # 10MB
LOG_LEVEL=DEBUG
```

this's the equivalent of the `config.json` file created with new dragon project.