Update to Go18

[Senyoret1]

Fixes #423

Changes:

• go.mod was added and the server should now work well with Go18.
• The readme was updated.
• The Travis file was updated to use the new Go version.

Does this change need to mentioned in CHANGELOG.md?
Yes

[Senyoret1]

The last commit includes the following changes:

• Now the Go server returns the correct mime type for JavaScript files, to make the app work well with modern brownser without an intermediate server.
• Now there are CLI params to configure the web server address:port, the URL of the Skycoin node and the path of the folder with the precompiled UI files. Run “go run explorer.go -h” to see the param list.
• The branch was updated.

[Senyoret1]

The last commit replaces gometalinter with golangci-lint. It works similar to how it works on the Skycoin wallet. Also, some changes were made to make the tests pass.

[Senyoret1]

The last commit inslcudes info in the readme and a new helper command 1n the makefile, for installing the linters in a local enviroment.

[0pcom]

looks really good, nicely done