allow manually specifying DPI

[simeonschaub]

PDFs are currently always read with the default DPI of 72, which is not particularly high. This allows manually specifying it using the dpi keyword. Still needs a test, is there a good sample PDF document I could use for this?

cc @timholy

[codecov[bot]]

Codecov Report

Base: 73.93% // Head: 66.96% // Decreases project coverage by -6.97% :warning:

Coverage data is based on head (1d11825) compared to base (079d62b). Patch coverage: 92.85% of modified lines in pull request are covered.

:exclamation: Current head 1d11825 differs from pull request most recent head 6fe954c. Consider uploading reports for the commit 6fe954c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #214 +/- ## ========================================== - Coverage 73.93% 66.96% -6.97% ========================================== Files 3 3 Lines 422 442 +20 ========================================== - Hits 312 296 -16 - Misses 110 146 +36 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/ImageMagick.jl/pull/214?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/libmagickwand.jl](https://codecov.io/gh/JuliaIO/ImageMagick.jl/pull/214?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2xpYm1hZ2lja3dhbmQuamw=) | `67.20% <90.90%> (+0.10%)` | :arrow_up: | | [src/ImageMagick.jl](https://codecov.io/gh/JuliaIO/ImageMagick.jl/pull/214?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL0ltYWdlTWFnaWNrLmps) | `83.87% <100.00%> (+0.31%)` | :arrow_up: | | [src/precompile.jl](https://codecov.io/gh/JuliaIO/ImageMagick.jl/pull/214?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL3ByZWNvbXBpbGUuamw=) | `0.00% <0.00%> (-76.93%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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[timholy]

LGTM, thanks!

No strong feelings about a test file, other than it should be reasonably small and with unproblematic license. Wikimedia has a few, this one seems appropriate: https://commons.wikimedia.org/wiki/File:FLAT_-_What_is_Creative_Commons.pdf