Add writesql+=  syntax support for func_odbc.conf

When i check my extensions conf, i have got an error:
`func_odbc.conf:6 E_CONF_KEY_INVALID: expected valid var/object or new context, got 'writesql+'`
My func_odbc.conf:
```
[ADD]
prefix=REPORTS
synopsis=Get report values from VAL1 and add it to the reports table
dsn=int-dsn-asterisk
writesql=INSERT INTO pbx.call_reports (values_list)
writesql+=VALUES ('${SQL_ESC(${VAL1})}')
```