add finder interface, rework chrome implementation, add examples

[srlehn]

This PR is in preparation for following browser implementations and adds:

• CookieStoreFinder interface for automatic location and standardized way to read cookies stored in different ways.
• add examples & changed README
• reworked chrome so that databases created on an OS different than the host OS can be read (fixes #13)
• add non-cgo alternative for keychain query on macos calling the security tool with the absolute path. (CGO_ENABLED=1 uses old cgo method) (fixes #6)
• internalized code for reuse by other browsers

type CookieStoreFinder interface {
    FindCookieStores() ([]CookieStore, error)
}

type CookieStore interface {
    ReadCookies(...Filter) ([]*Cookie, error)
    Browser() string
    Profile() string
    IsDefaultProfile() bool
    FilePath() string
    Close() error
}

[roberChen]

@srlehn Thanks for great works! @zellyn Can you take a look at this pr?