Prevent over stripping a drive letter in Windows

[kai2nenobu]

This PR let use and deactivate command use an OS-specific path list separator instead of :.

This resolves #335.

About test

Existing tests fail in Windows.

Existing test result in windows on master (dcf0285)

> ver

Microsoft Windows [Version 10.0.18362.356]
> go version
go version go1.13 windows/amd64
> git rev-parse HEAD
dcf02850eddf464edb4c47495c7d1d79a9820832
> go test ./...
?       github.com/kai2nenobu/jabba     [no test files]
?       github.com/kai2nenobu/jabba/cfg [no test files]
--- FAIL: TestDeactivate (0.00s)
    deactivate_test.go:26: actual: [export PATH="/usr/local/bin:C:\Users\kai2nenobu\.jabba/jdk/zulu@1.8.72/bin:/system-jdk/bin:/usr/bin" export JAVA_HOME="/system-jdk" unset JAVA_HOME_BEFORE_JABBA] != expected: [export PATH="/usr/local/bin:/system-jdk/bin:/usr/bin" export JAVA_HOME="/system-jdk" unset JAVA_HOME_BEFORE_JABBA]
--- FAIL: TestUse (0.00s)
    use_test.go:47: actual: [export PATH="C:\Users\kai2nenobu\.jabba\jdk\1.7.2\bin;/usr/local/bin:C:\Users\kai2nenobu\.jabba/jdk/1.6.0/bin:/usr/bin" export JAVA_HOME="C:\Users\kai2nenobu\.jabba\jdk\1.7.2" export JAVA_HOME_BEFORE_JABBA="/system-jdk"] != expected: [export PATH="C:\Users\kai2nenobu\.jabba/jdk/1.7.2/bin:/usr/local/bin:/usr/bin" export JAVA_HOME="C:\Users\kai2nenobu\.jabba/jdk/1.7.2" export JAVA_HOME_BEFORE_JABBA="/system-jdk"]
FAIL
FAIL    github.com/kai2nenobu/jabba/command     0.606s
ok      github.com/kai2nenobu/jabba/command/fileiter    (cached)
ok      github.com/kai2nenobu/jabba/semver      (cached)
?       github.com/kai2nenobu/jabba/w32 [no test files]
FAIL

So I added new tests and let existing tests skip in Windows. However, I'm not familiar with golang testing, please review test code and let me know if there are smarter manners.

I tested this PR in travis ci on three OSs (linux, macos, windows) and two golang versions (1.9, 1.13). See travis ci log for more details.