feat: preserve formatting when writing to package.json

[alasdairhurst]

This PR implements a feature (or maybe it's a fix?) to get standard-version to preserve the indentation and
line feed of package.json instead of overriding the user's preferred format with 2 space indentation. (See #43)

detect-indent and detect-newline are used to figure out the current indentation of package.json. This is done in an identical way to how npm handles writing changes to package.json.

Notable internal change:

1. package.json is no longer loaded with require(). While it should be functionally identical, in order to detect the formatting, the file to be read in as plain text. This caused certain tests to fail since the require cache wasn't updated during the bump, resulting in additional places where the cache needed to be deleted between invocations of the CLI in these tests. By using fs.readFileSync() in both places in the code where package.json is read instead of require(), the deletion of the require cache in the tests is no longer necessary for them to work.

[coveralls]

Coverage remained the same at 100.0% when pulling 856a5c5 on alasdairhurst:preserve-formatting into 844cde6 on conventional-changelog:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 856a5c5 on alasdairhurst:preserve-formatting into 844cde6 on conventional-changelog:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 856a5c5 on alasdairhurst:preserve-formatting into 844cde6 on conventional-changelog:master.

[bcoe]

@alasdairhurst sorry this took so long to land, thank you for the contribution.

If you don't mind, give this a try:

npm i standard-version@5.0.0

You should find it has your changes.