Make SVGs valid while device is open

[noamross]

This PR institutes changes that allow an SVG to be viewed while the device is open.

• First, the file stream is flushed after an R graphics callback.
• Secondly, the file always ends with </svg>, making it a valid SVG while the device is open.
• Finally, the option overwrite_page = TRUE allows the current file to be overwritten when a new page is created. Default is FALSE, which uses the current behavior.

This can easily be made compatible with #46 if need be.

[codecov-io]

Current coverage is 92.62%

Merging #53 into master will decrease coverage by -0.31% as of 3c4b0f6

@@            master     #53   diff @@
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  Files            6       6       
  Stmts          382     407    +25
  Branches         0       0       
  Methods          0       0       
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+ Hit            355     377    +22
  Partial          0       0       
- Missed          27      30     +3

Review entire Coverage Diff as of 3c4b0f6

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