Make it possible to change the port of Local DynamoDB server

[ishanthilina]

• [x] I have searched the issues of this repository and believe that this is not a duplicate. (no other issues at the moment)

This is an improvement for the existing functionality.

When trying to connect to the local DynamoDB server, there is an assumption that the server is running on port 8000. But the port Local DynamoDB server runs is configurable, thus this assumption is not always true.

Expected Behavior

It should be possible to change the port of the local DynamoDB server which Dynamon tries to connect to.

Current Behavior

It's not possible to configure this port.

Steps to Reproduce

1. Start local DynamoDB server in a port other than 8000.
2. Try to connect to it using Dynamon.

Your Environment

Tech	Version
OS version	Mac OS High Sierra 10.13.4
Dynamon version	0.1.5

[deptno]

It need UI, logic part.

click plus icon

then show user input prompt like this(just prompt, it's ok)

window.prompt('localhost port')

I'm waiting someone makes a PR or I'll do later.