Changes in version 0.1.0

* add `print.GlobalOptionsFun()` and `dump_opt()`.
* add `.DollarNames.GlobalOptionsFun()` to allow auto completion after `$`.
* add `.v()` and `.v$..` to access other option values when configuring 
  current option.
* add `set_opt()` to replace `setGlobalOptions()`
* add `ADD` argument in the option function to add new options after the 
  option function is created.
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Changes in version 0.0.13

* register S3 method in NAMESPACE

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Changes in version 0.0.12

* add .synonymous option

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Changes in version 0.0.11

* improved docs
* opt$nm can be set to NULL now
* `failed_msg` are wrapped

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Changes in version 0.0.10

* `$<-GlobalOptionsFun` are now invoked in right environment

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Changes in version 0.0.9
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* support local mode

Changes in version 0.0.8
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* assign a class for the generated functions
* option value can be assigned by '$' symbol

Changes in version 0.0.6
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* re-construct by reference class
* default value for `.filter` and `.validate` are set to `NULL`
* private value will not be reset if calling environment is not same as generating
  environment

Changes in version 0.0.5
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* change interval options to `__generatedNamespace__`

Changes in version 0.0.4
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* completely replace `is` with `inherits`

Changes in version 0.0.3
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* remove dependency of `methods` package
* `value_fun` is deleted after setting every option
* function is attached to the value if option values is set as a dynamic function
  so that opt(opt(READ.ONLY = FALSE)) will be correct if there are option values 
  set as dynamic functions.

Changes in version 0.0.2
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* use knitr to build the vignette.
* add .private and .visible fields for making options specification. Private options
  can only be modified if the namespace in which the options function is generated and the
  namespace where the options function is called is the same namespace. Invisible options
  will not be returned when getting the option values.

Changes in version 0.0.1
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* the first release
