add a reactlog_enable()/reactlog_disable() instead of having to set the R option

[daattali]

Fixes #61

[schloerke]

Thank you for the PR, @daattali !

Could we change reactlog_set() to two functions: reactlog_enable() and reactlog_disable(warn = TRUE)?

With reactlog_disable(warn = TRUE), it should display a warning like "Restart R to free up 'reactlog' memory" if isTRUE(warn)`

(Feel free to put them under the same Rd file)

[daattali]

@dtkaplan this is for you!

[schloerke]

Thank you!

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