Add support for reading files

[vlovgr]

Add file and fileWithName methods for reading the contents of files, optionally modifying the read file contents before attempting the type conversion, and optionally specifying a charset. Note that this is currently only implemented for the JVM. Also adds the following in support of the functionality: ConfigKeyType#File, ConfigSource#File, ConfigReader#mapEntryValue, ConfigSourceEntry#mapValue.

❯ cat /tmp/num.txt
3

scala> import ciris._
import ciris._

scala> fileWithName[Int]("/tmp/num.txt")
res0: ciris.ConfigValue[Int] =
ConfigValue(Left(WrongType((/tmp/num.txt,UTF-8), 3
, Int, ConfigKeyType(file), Some(java.lang.NumberFormatException: For input string: "3
"))))

scala> fileWithName[Int]("/tmp/num.txt", _.trim)
res1: ciris.ConfigValue[Int] = ConfigValue(Right(3))

[codecov]

Codecov Report

Merging #69 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #69   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          28     31    +3     
  Lines         433    446   +13     
  Branches        3      3           
=====================================
+ Hits          433    446   +13

Impacted Files	Coverage Δ
...ore/shared/src/main/scala/ciris/ConfigSource.scala	100% <ø> (ø)	⬆️
...les/core/shared/src/main/scala/ciris/package.scala	100% <ø> (ø)	⬆️
...re/shared/src/main/scala/ciris/ConfigKeyType.scala	100% <ø> (ø)	⬆️
...in/scala/ciris/ConfigKeyTypePlatformSpecific.scala	100% <100%> (ø)
...hared/src/main/scala/ciris/ConfigSourceEntry.scala	100% <100%> (ø)	⬆️
...ore/shared/src/main/scala/ciris/ConfigReader.scala	100% <100%> (ø)	⬆️
...ain/scala/ciris/ConfigSourcePlatformSpecific.scala	100% <100%> (ø)
...m/src/main/scala/ciris/CirisPlatformSpecific.scala	100% <100%> (ø)
.../shared/src/main/scala/ciris/ConfigException.scala	100% <0%> (ø)	⬆️
... and 2 more

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