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getv accepts a default value

The `getv` template function does not accept a default value. The
workaround is to write templates like this:

    {{if exists "/example/app/option"}}
    option = {{getv "/example/app/option"}}{{else}}default_value{{end}}

Resolve the issue by accepting an optional default value. Templates
can now be written like this:

    value: {{getv "/key" "default_value"}}

This change is backwards compatible with the current `getv` function.
If the key is missing and a default value is given the missing key error
will be suppressed and the default value will be returned.

If the key is missing and no default value is given the `getv` function
will return an error and template processing will halt.

Fixes kelseyhightower#219

Merge pull request kelseyhightower#394 from HeavyHorst/master

update some docs

Capture and return errors during watches on consul

Before this change consul errors were not returned correctly
during watches. Resolved the issue by returning the initial
watch index and error and avoid the nil pointer dereference.

Fixes kelseyhightower#322

bump to v0.11.0

bump version to v0.10.0

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