Reflow long comment lines which don't end with a period

[shivansh]

$ cat x.go
package main

// Unlike the above comment block, only the current line in this comment block has length greater than the
// target maximum line length
// but since the previous line doesn't end with a period, it should be reflown with this line
// despite both the lines being shorter than the target maximum line length.

$ golines --shorten-comments x.go
package main

// Unlike the above comment block, only the current line in this comment block has length greater
// than the
// target maximum line length
// but since the previous line doesn't end with a period, it should be reflown with this line
// despite both the lines being shorter than the target maximum line length.

The modified behavior is to reflow long comment lines which don't end
with a period with the consecutive comment lines, irrespective of the
latter being long or not.

$ golines --shorten-comments x.go
package main

// Unlike the above comment block, only the current line in this comment block has length greater
// than the target maximum line length but since the previous line doesn't end with a period, it
// should be reflown with this line despite both the lines being shorter than the target maximum
// line length.

[shivansh]

Though the comment formatting is still in an experimental stage, I believe this PR will improve it to some extent. However, if this is accepted, a note should be added in README for users to be aware of the usage of period (.) to avoid unexpected behavior.
For example -

$ cat x.go 
package main

// - this is a long comment this is a long comment this is a long comment this is a long comment this is a long comment.
// - this is a short comment

$ golines --shorten-comments x.go
package main

// - this is a long comment this is a long comment this is a long comment this is a long comment
// this is a long comment - this is a short comment

The above unexpected behavior can be avoided by adding a . at the end of first bullet:

$ cat x.go 
package main

// - this is a long comment this is a long comment this is a long comment this is a long comment this is a long comment.
// - this is a short comment

$ golines --shorten-comments x.go
package main

// - this is a long comment this is a long comment this is a long comment this is a long comment
// this is a long comment.
// - this is a short comment