Support the setting of 'maven.executable.path' in vscode workspace folder settings

[bsaby]

Describe the bug
Open a multi-root workspace with multiple maven projects, I want one of the projects to use its own mvnw file and others use the environment maven path. But when I search using 'maven' keyword in specified folder settings, there are only two maven settings and I can't find 'maven.executable.path'

To Reproduce

1. Open a muitl-root worksapce
2. Open folder settings to choose a project and search maven related settings

Environments (please complete the following information as much as possible):

• OS: Windows 10/OS X
• VS Code version: 1.25.1
• Extension version 0.9.2

Screenshots

[Eskibear]

fixed in #88