[SQLLINE-386] support working with saved connections

[snuyanzin]

1. Path to file with connections could be specified in connectionConfig property
2. Connections in a file have names, these names could be used as connection nicknames
3. Connections allowed for both: input args and !connect command
4. !resetconfconnections !rereadconfconnections to reset connections loaded from file or use another file
5. !confconnections !showconfconnections to read current connection file content

example of usage !connect -c con_name where con_name or !connect -cn con_name and then con_name will be also used as a nickname.
It is still possible to specify properties, the full rule is

Usage: connect [-p property value]* (-(c|cn) <connectionName> | <url>) [username] [password] [driver]

the content of connection file is like:

pg:
  url: jdbc:postgresql://localhost:5432/postgres
  user: postgres
  password: postgres
mysql: # comment
  url: jdbc:mysql://localhost/mysql?autoReconnect=true
  user: mysql
  password: mysql
pg2:
  url: jdbc:postgresql://localhost:5432/postgres2
  user: postgres2
  password: postgres2

name of connection on a separate line, then connection properties with indentation (number of does not matter but must be same for all properties within connection).
comments are supported only on a separate line and on a connection name line

should fix #386

[snuyanzin]

rebased and added doc

[snuyanzin]

resolved merge conflicts, rebased