Do not add entries that already exist

[jugglinmike]

Modification of the system hostfile requires administrative privileges,
but some users may not want to grant those rights to this plugin. The
plugin already scans the hosts file for previously-existing entries, but
the heuristic is very strict and only matches machine-generated entries
as produced by the plugin itself.

Allow consumers to opt out of automatic hostfile modification (thereby
avoiding the need to grant additional privileges) by relaxing the
requirements for "matching" entries. Update the status messages to
document this behavior.

This resolves gh-69.

[cgsmith]

I like this change :). I had issues before where the checks would add multiple entries do to vagrant destroy not properly hooking in.

Ill test this out and report back. Anyone else reading feel free to test and comment. Thanks @jugglinmike

[QWp6t]

In Windows hosts files (and possibly others, I don't know), you can have multiple host names on one line.

192.168.56.101    www.example.com example.com

This would fail if it looked for hostEntryPattern("192.168.56.101", "example.com") because the preceding www.example.com does not match \s+ in the regex.

[cgsmith]

Merged in and tested - preparing for release.